Depends on so many things. Are they adults? What's the temperature? Do they get along well or is one or more shunned away from the food? Do they come out to feed right away and get them when they are all in a pile or do they hide until you don't watch anymore and then have to forage for all of their flies? Is your enclosure completely fly-proof?
Having said all of that, the answer is 413. No, just kidding I have no idea. What I would do is feed them and pay attention to how much they eat in one sitting. It's ok if some escape - it gives them something to chase down later. I feed my frogs every other day and if I still see flies in the tank when I am getting ready to feed again, I back off the amount from what I fed them last time a little bit. You want your frogs to be getting as many dusted flies as possible and to be nice and hungry (NOT starving/emaciated) when the next flies come in. That means they should finish off most of the flies before the next feeding. The longer it is between when you feed the flies and when they get eaten, the less dusting supplement (I suggest Repashy Calcium Plus) the frogs get.
I think of fly amounts in teaspoons, tablespoons, etc. rather than individual flies. Hard to wrap your head around a number like that and you end up psyching yourself out.
I have 5 Azureus (±8 months oow, so sub-adults) in a tank and they are voracious little monsters. I'm still feeding them melanos because all my others darts are thumbnails and I haven't taken the plunge to start any hydei cultures yet. I feed every other day and just dump a bunch on the ground wherever they frogs are at. I'd say each one gets about a dozen or so flies right off the bat. If any escape the immediate devouring, they are soon sought out and gobbled up. I never see any leftover flies. And the off days I don't feed, the frogs are constantly foraging for some.
Heck today was an off day and when I came by their tank, 2 of them jumped right up to the doors begging me for flies!