I am adding an A/C to my garage and need to add a 240V circuit. The
loadcenter is conveniently in my garage and has plenty of open spaces.
However, I want to make sure I am not overloading it.

What are the rules for adding circuit breakers? The label on the
loadcenter says 120Amp max. but it already has 12 20Amp circuit breakers
in it. 12X20 = 240Amps which is twice the rating. So, I am a bit
confused.