Before discussing MIDI note numbers, it is important to understand some basic principles about how musicians name musical notes. So, it's traditional to name musical pitches based upon the piano keyboard. A piano has white and black keys. They are visually grouped into octaves where one octave contains 12 keys. Below is a picture of one octave of keys upon a piano.
Musicians append sharps or flats to the alphabetical names to identify the black keys.
Below is a portion of a piano with the keys labeled as a musician would refer to them. The numbers used are 0 to 127. As mentioned, the lowest note upon a MIDI controller is a C and this is assigned note number 0. The D note above that would have a note number of 2. So, a MIDI note number of 69 is used for A440 tuning.