well if you dont have anything in the tank that will eat them then i would suggest no more than a dozen cuz they will breed. i threw a handful in my 125 planted community tank and now they are over an inch long. pretty cool little shrimp actually.
make sure you will have alot of hiding places for them or else that dozen will be a dozen of tasty treats for the mollies.. Also ghost shrimps are hard to reproduce since they need specific water conditions to spawn
i know this is OLD, but ghost shrimp do not need specific water conditions to reproduce. thats amano shrimp, and they need brackish water.
as with amano shrimp however, ghost shrimp also have a larval stage and need green water to feed on (within hours of hatching or they die). so they are semi hard to breed, but its not impossible.