Steve,
How many flies to feed depends on whether you are feeding melanogaster or hydei, and whether your frogs are adults or froglets. I used to feed my froglets about 10-15 melanogaster each 2x per day (dusted with Rep-Cal and Herptivite mix). If they are eating hydei, you can probably get away with only feeding about 5 flies each 2x per day. My adult tinc gets about 20-30 hydei once per day (still dusted each time).
Micro crickets are certainly acceptable fare, but don't overfeed those, as crickets left in the tank can quickly grow to be too big to be eaten. Springtails are another good cultured food to add variety, as are flour beetle larvae (look like TINY and soft mealworms--super easy to culture).
You might have to watch out for obesity in adults, but you really want to pour the food on with the juvi's to ensure proper development and rapid growth. They need to look quite plump. It's absolutely amazing how much chow those little guys can put down!
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Homer W. Faucett III, esq.
Purveyor of Trivialities and Fine Nonsense