i keep my tank stocked with about twenty feeder guppies at all times but i have three hatchlings in there so i would think that as long as there is about ten in with your turtle he/she will be fine. remember they like to laze about and wait for the food to come to them so keep that in mind. also what size tank are you keeping him in. that can have a lot to do with how many feeders you should keep in with him/her.