At 12-3/4" most bowls will be out from the wall. Which I perfer over having the tank touch the wall.
Some toilets have less space behind the tank then others.
Most standard 12" toilets will have between 1/2" and 1-1/8" behind the tank at 12"
Something like a TOTO Drake II would have 1/2" behind the tank at 12", but then that's an elongated bowl.
Is it the space behind, or how far it comes out into the room?