There's no exact amount... it really depends on how old and large your nymph is. When I got mine as a presumed L3, she was eating five or six fruit flies every other day, but on the next molt she ate about ten a day, and after that, 20 or so twice a day!
Just try dumping a few in there and you'll eventually learn how much is enough for your nymph. You'll want to make sure the abdomen is nice and round, but not to the extent that it's folding or seems to be close to bursting. I usually let my mantis get thin again for a few days before I feed, just to make sure I don't overfeed.