I played instant action with 11 players and I got respawn killed all the time. I killed people who respawned too, because that was the only way to kill people. Especially skyline is to small for 12 players.
12 is fine. Black Ops works with 12 on smaller maps than Crysis 2 has. The only thing which needs to be fixed is the terrible spawning. I can't tell you how many times I've spawned and been shot in the back seconds later.