How many bullets are in a M-16 banana magazine?

They aren't refered to as 'banana magazine'.

People will laugh at you for saying that. Not to be rude.

But some hold 20, some 30.

When Armalite first produced the AR-15, the magazines were straight walled. This limited the capacity to 20 rounds, as the taper in the cartridge case would cause feeding problems. To counteract this, the "banana magazine" as you called it was produced. In USA conversation, this term is not generally used, but has been used in the media and some movies. 20 round magazines today are still straight walled.

I think you are talking about the STANAG magazine, which holds up to 20 rounds