The way I play is that if a word sucks (like “ch”) you don’t use it unless if helps you do a 2 or 3-way word that is not trivial.
Cecil’s updated answer is still incorrect.
The official list in the US has 101 2-letter words.
The official list in the UK has 124 2-letter words.
Nitpick: The major scale is in the chromatic scale.