Er- as many as will fit? Or do you mean to start? You could probably dump a thousand in a 10 and have a nice colony set up. I have mine in one of those critter keeper things which is much smaller than a 10g. I'm not sure on exact numbers, but there are several hundred insects in there at least.
I was wondering how many mealies could be put in a 10 gal.?(maximum) how man beetles could be put in a 10 gal.? (maximum) or can they be like piled on one another? i have 33 gal. rubbermaid tub..
I have had at least 3000 in a small kritter keeper (6"x12"x9") and at least several hundred beetles in the same small container. I easily keep 1000 crickets in a 10 gallon. I can't imagine you could accumulate too many of either feeder for a 10 gallon.