I've heard 1 gallon per frog, and 2 gallons per frog, for ADFs. There's no real set rule to it, near as I can tell, since opinions obviously vary.
Either way, the more room they have, the more active they are, and in a 2.5Gal, the more frogs you have, the less room they have. I'm told they're fine alone. I own more than one, and it's true that I don't see much intentional interaction now, while they're juvenille. They do love to swim, they're pretty active little guys once they've settled in.
I wouldn't put more than 2 in there, personally. I just have the 'less is more' mentality.
Or if you like, you can put one betta in there.
But if you look at my post count, most of my posts are in the Betta forum, so I'm obviously a biased opinion.