Question: How many stable isotopes of chlorine are there?
About the article "Proton", section "Atomic number".

Candidate passages:
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 
Fluorine has one stable and naturally occurring isotope, fluorine-19.
However, there are trace amounts in nature of the radioactive isotope fluorine-23, which occurs via cluster decay of protactinium-231.
A total of eighteen isotopes of fluorine have been discovered, with atomic masses ranging from 14 to 31.
Chlorine has two stable and naturally occurring isotopes, chlorine-35 and chlorine-37.
However, there are trace amounts in nature of the isotope chlorine-36, which occurs via spallation of argon-36.
A total of 24 isotopes of chlorine have been discovered, with atomic masses ranging from 28 to 51.
There are two stable and naturally occurring isotopes of bromine, bromine-79 and bromine-81.
A total of 32 isotopes of bromine have been discovered, with atomic masses ranging 67 to 98.
There is one stable and naturally occurring isotope of iodine, iodine-127.
However, there are trace amounts in nature of the radioactive isotope iodine-129, which occurs via spallation and from the radioactive decay of uranium in ores.
Several other radioactive isotopes of iodine have also been created naturally via the decay of uranium.
A total of 38 isotopes of iodine have been discovered, with atomic masses ranging from 108 to 145.
There are no stable isotopes of astatine.
However, there are three naturally occurring radioactive isotopes of astatine produced via radioactive decay of uranium, neptunium, and plutonium.
These isotopes are astatine-215, astatine-217, and astatine-219.
A total of 31 isotopes of astatine have been discovered, with atomic masses ranging from 193 to 223. 
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. 
Of the 80 elements with a stable isotope, the largest number of stable isotopes observed for any element is ten (for the element tin).
No element has nine stable isotopes.
Xenon is the only element with eight stable isotopes.
Four elements have seven stable isotopes, eight have six stable isotopes, ten have five stable isotopes, nine have four stable isotopes, five have three stable isotopes, 16 have two stable isotopes (counting as stable), and 26 elements have only a single stable isotope (of these, 19 are so-called mononuclidic elements, having a single primordial stable isotope that dominates and fixes the atomic weight of the natural element to high precision; 3 radioactive mononuclidic elements occur as well).
In total, there are 254 nuclides that have not been observed to decay.
For the 80 elements that have one or more stable isotopes, the average number of stable isotopes is 254/80 = 3.2 isotopes per element. 
The mass number should also not be confused with the relative atomic mass (also called atomic weight) of an element, which is the ratio of the average atomic mass of the different isotopes of that element (weighted by abundance) to the unified atomic mass unit.
This weighted average can be quite different from the near-integer values for individual isotopic masses.
For instance, there are two main isotopes of chlorine: chlorine-35 and chlorine-37.
In any given sample of chlorine that has not been subjected to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37.
This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/mol).
Moreover, the weighted average mass can be near-integer, but at the same time not corresponding to the mass of any natural isotope.
For example, bromine has only two stable isotopes, 79Br and 81Br, naturally present in approximately equal fractions, which leads to the standard atomic mass of bromine close to 80 (79.904 g/mol), even though the isotope 80Br with such mass is unstable.