If you're thinking tree frogs I wouldn't go anything bigger than a pair of grays in a 20 gallon, & you'll need to vertical to keep them happy & healthy. If you go toads I would say go with a smaller species like the Bumble Bee or Western Green unless you plan on only having one. Warning – one toad will lead to many more, they have the ability to connect with your soul turning you into a toad lovin' fool .