Question: How many isotopes of chlorine have 18 or 20 neutrons?
About the article "Proton", section "Atomic number".

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Chlorine-37, or , is one of the stable isotopes of chlorine, the other being chlorine-35 ().
Its nucleus contains 17 protons and 20 neutrons for a total of 37 nucleons.
Chlorine-37 accounts for 24.23% of natural chlorine, chlorine-35 accounting for 75.77%, giving chlorine atoms in bulk an apparent atomic weight of .
Remarkably, the Solar neutrinos were discovered by an experiment using a radiochemical method based on Chlorine-37 transmutation. 
A nucleogenic isotope or nuclide, is one that is produced by a natural terrestrial nuclear reaction, other than a reaction beginning with cosmic rays (the latter nuclides by convention are called by the different term cosmogenic).
The nuclear reaction that produces nucleogenic nuclides is usually interaction with an alpha particle or the capture of fission or thermal neutron.
Some nucleogenic isotopes are stable and others are radioactive.
An example of a nucleogenic nuclide would be neon-21 produced from neon-20 that absorbs a thermal neutron, to become stable neon-21 (some neon-21 is also primordial).
Other nucleogenic reactions that produce heavy neon isotopes are (fast neutron capture, alpha emission) reactions, starting with magnesium-24 and magnesium-25, respectively.
The source of the neutrons in these reactions is often secondary neutrons produced by alpha radiation from natural uranium and thorium in rock.
Because nucleogenic isotopes have been produced later than the birth of the solar system (and the nucleosynthetic events that preceded it), nucleogenic isotopes, by definition, are not primordial nuclides.
However, nucleogenic isotopes should not be confused with much more common radiogenic nuclides that are also younger than primordial nuclides, but which arise as simple daughter isotopes from radioactive decay.
Nucleogenic isotopes, as noted, are the result of a more complicated nuclear reaction, although such reactions may begin with a radioactive decay event.
Alpha particles that produce nucleogenic reactions come from natural alpha particle emitters in uranium and thorium decay chains.
Neutrons to produce nucleogenic nuclides may be produced by a number of processes, but due to the short half-life of free neutrons, all of these reactions occur on Earth.
Among the most common are cosmic ray spallation production of neutrons from elements near the surface of the Earth.
Alpha emission produced by some radioactive decay also produces neutrons by spallation knockout of neutron rich isotopes, such as the reaction of alpha particles with oxygen-18.
Neutrons are also produced by the neutron emission type of radioactive decay, and also from spontaneous fission of fissile elements on Earth, particularly uranium-235.
Nucleogenesis (also known as nucleosynthesis) as a general phenomenon is a process usually associated with production of nuclides in the Big Bang or in stars, by nuclear reactions there.
Some of these neutron reactions (such as the r-process and s-process) involve absorption by atomic nuclei of high-temperature (high energy) neutrons from the star.
These processes produce most of the chemical elements in the universe heavier than zirconium (element 40), because nuclear fusion processes become increasingly inefficient and unlikely, for elements heavier than this.
By convention, such heavier elements produced in normal elemental abundance, are "not" referred to as "nucleogenic."
Instead, this term is reserved for nuclides (isotopes) made on Earth from natural nuclear reactions.
Also, the term "nucleogenic" by convention excludes artificially produced radionuclides, for example tritium, many of which are produced in large amounts by a similar artificial processes, but using the copious neutron flux produced by conventional nuclear reactors. 
 in chemistry.
Atomic number.
In chemistry, the number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs.
For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms.
The chemical properties of each atom are determined by the number of (negatively charged) electrons, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero.
For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl− anion has 17 protons and 18 electrons for a total charge of −1.
All atoms of a given element are not necessarily identical, however, as the number of neutrons may vary to form different isotopes, and energy levels may differ forming different nuclear isomers.
For example, there are two stable isotopes of chlorine: with 35 − 17 = 18 neutrons and with 37 − 17 = 20 neutrons.
Elements are composed of one or more naturally occurring isotopes.
The unstable (radioactive) isotopes are either primordial or postprimordial.
Primordial isotopes were a product of stellar nucleosynthesis or another type of nucleosynthesis such as cosmic ray spallation, and have persisted down to the present because their rate of decay is so slow (e.g., uranium-238 and potassium-40).
Postprimordial isotopes were created by cosmic ray bombardment as cosmogenic nuclides (e.g., tritium, carbon-14), or by the decay of a radioactive primordial isotope to a radioactive radiogenic nuclide daughter (e.g., uranium to radium).
A few isotopes are naturally synthesized as nucleogenic nuclides, by some other natural nuclear reaction, such as when neutrons from natural nuclear fission are absorbed by another atom.
As discussed above, only 80 elements have any stable isotopes, and 26 of these have only one stable isotope.
Thus, about two thirds of stable elements occur naturally on Earth in multiple stable isotopes, with the largest number of stable isotopes for an element being ten, for tin ().
There are about 94 elements found naturally on Earth (up to plutonium inclusive), though some are detected only in very tiny amounts, such as plutonium-244.
Scientists estimate that the elements that occur naturally on Earth (some only as radioisotopes) occur as 339 isotopes (nuclides) in total.
Only 254 of these naturally occurring isotopes are stable in the sense of never having been observed to decay as of the present time.
An additional 35 primordial nuclides (to a total of 289 primordial nuclides), are radioactive with known half-lives, but have half-lives longer than 80 million years, allowing them to exist from the beginning of the solar system.
See list of nuclides for details.
All the known stable isotopes occur naturally on Earth; the other naturally occurring-isotopes are radioactive but occur on Earth due to their relatively long half-lives, or else due to other means of ongoing natural production.
These include the afore-mentioned cosmogenic nuclides, the nucleogenic nuclides, and any radiogenic radioisotopes formed by ongoing decay of a primordial radioactive isotope, such as radon and radium from uranium.
An additional ~3000 radioactive isotopes not found in nature have been created in nuclear reactors and in particle accelerators.
Many short-lived isotopes not found naturally on Earth have also been observed by spectroscopic analysis, being naturally created in stars or supernovae.
An example is aluminium-26, which is not naturally found on Earth, but is found in abundance on an astronomical scale.
The tabulated atomic masses of elements are averages that account for the presence of multiple isotopes with different masses.
Before the discovery of isotopes, empirically determined noninteger values of atomic mass confounded scientists.
For example, a sample of chlorine contains 75.8% chlorine-35 and 24.2% chlorine-37, giving an average atomic mass of 35.5 atomic mass units.
According to generally accepted cosmology theory, only isotopes of hydrogen and helium, traces of some isotopes of lithium and beryllium, and perhaps some boron, were created at the Big Bang, while all other isotopes were synthesized later, in stars and supernovae, and in interactions between energetic particles such as cosmic rays, and previously produced isotopes.
(See nucleosynthesis for details of the various processes thought responsible for isotope production.)
The respective abundances of isotopes on Earth result from the quantities formed by these processes, their spread through the galaxy, and the rates of decay for isotopes that are unstable.
After the initial coalescence of the solar system, isotopes were redistributed according to mass, and the isotopic composition of elements varies slightly from planet to planet.
This sometimes makes it possible to trace the origin of meteorites. 
Chlorine (Cl) has 24 isotopes with mass numbers ranging from 28Cl to 51Cl and 2 isomers (34mCl and 38mCl).
There are two principal stable isotopes, 35Cl (75.78%) and 37Cl (24.22%), found in the relative proportions of 37.89:12.11, respectively, giving chlorine a standard atomic mass of 35.453.
The longest-lived radioactive isotope is 36Cl which has a half-life of 301,000 years.
All other isotopes have half-lives under 1 hour, many less than one second.
The shortest-lived are 29Cl and 30Cl, with half-lives less than 20 and 30 nanoseconds, respectively—the half-life of 28Cl is unknown.
Standard atomic mass: 35.453(2) u