Subject: questions regarding mRNA codons

I have a couple of questions regarding genetic transcription/translation.  
I know that there is a start codon on DNA and that after this, the mRNA 
begins reading the DNA and then this message is translated to protein 
synthesis via tRNA, using triplet codons.  My first question is how did 
scientists originally figure out that three bases coded for an amino acid, 
and not two or four or some other number?  Also, I know that 20 amino acids 
are coded for.  Do more amino acids exist?  I thought that the number of 
amino acids present in organisms was actually much higher.  If so, how are 
these synthesized?  And what about base triplets that are not listed as 
coding for start, stop, or an amino acid?  Are they also stop codons?  
Thank you for your time.

Re: questions regarding mRNA codons