There are only 14k, but they are very unevenly distributed, so you might be surprised how many are left for the taking: https://gwern.net/tla

You only have to go as far as 'CQK' before there is, AFAICT and based on Wikipedia entries, essentially no even slightly important use of a TLA. There are then another >2k more or less unused, and if you go to four-letters, you've got ~390k to play with.