btw there is no "unlimit" function for turrets.
So you have to use the trick with 999 turrets. You also can type 99999999999999999999 turrets, but:
DO YOU EVER BUILD 999 TURRETS IN GAME?
even the strongest server is probably not able to handle 9999 turrets anyway. they would cause huge lags, making everything unplayable - or even crash the server completely. so why should I create the illusion that it is possible to have infinite turrets? it's not possible of course!
you have to consider that each turret has to scan the environment for close enemies ALL THE TIME. this takes muuuuch CPU power when you sum it up for many turrets + players.
that's why turrets are the buildings which take the most CPU time by far. so be careful with them... also note that this limit is PER PLAYER! a limit of 10 would already lead to up to 320 turrets in total on a 32 player server!
and yeah.. 320 turrets which check for close enemies all the time.. ouch!
Oh to add onto DC's rant--even if your server can support it, when a lot of turrets fire at once it will lag players and possibility lag them out of the game.