There is 1 codon for "start." It is AUG, which is the amino acid Methionine.
There are 3 "stop" codons. They are UAA, UAG, and UGA.
A 3 base code could specify a maximum of 64 amino acids, so 64-4 start and stop codons = 60 amino acids COULD be coded for. However, there are only 20 amino acids, so most amino acids have more than one sequence that codes for them. (example: The amino acid serine can be coded for with AGU or AGC).
When scientists were trying to figure out the code, they at first thought that each base coded for an amino acid, but with only 4 bases, 4 X 1 = 4 would only code for 4 amino acids. If the code was 2 bases 4 X 4 = 16 it could only code for 16 amino acids, which is still not enough. With a 3 base code 4 X 4 X 4 = 64, which is more than enough to code for the 20 amino acids. George Gamow is the name of the man that came up with the idea of the 3 base code.