I started out with 300 and now have probably over 1000... I saw 4 or 5 of the snails on the glass all pooping out white strands of eggs at the same time one day... So yes, evidently when things are healthy they reproduce like mad.. I'm going to start trading / selling mine to local reefers.. I coul probably sell off around 500 and not make a dent in them...
I have a 230g tank. I put none of the ceriths in the sump, and there's easily over 20 in the sump now... Eggs must have made it into the overflows... heh.
There's lots of stuff for them to clean, and eat, but, yeah, there's a point at which they'll starve and die too from lack of food.