My guess is that the styrofoam will insulate the glass but it will also keep enough heat from getting through. You might need a heat lamp/ heat emitter instead depending on how cold your house gets.

Personally I'd say 3-5 frogs would fit. There's a ten gallon minimum per frog rule of thumb, but you have to consider that White's get big, especially females. They aren't especially territorial so they should do okay in groups as long as they have plenty of hiding spots. But one thing to consider is that more frogs= more poop. More frogs per gallon= more frequent cleaning of the tank.