It really depends on what you want. I have a 10 gallon divided three way, a Betta in each section with a heater and filter. And one my Betta is in a 2.5 gallon kritter keeper with a heater and he is doing great.
Bettas: Rose, Sophia, Addie, Ghost, Buster, Holt, Aidan, and Mercutio!
Just my animals: 0.2 Cornsnakes, 0.2 Bearded Dragons, 1.0 Water Dragon, 2.2 Crested Geckos, 1.0 Tiger Salamander, 0.0.1 Tadpole, 0.0.2 Red Eared Slider Turtles, 0.0.1 Map Turtle, 0.0.2 Hermit Crabs, 1.0 Hamster, 1.0 Rabbit, 0.1 Horses, 2.2 Dogs, 0.2 Cats, Chickens, Ducks, Oscar Tank, Planted Bowl, 10 Gallon, 4x 0.5 Gallon, 5 Gallon, and Planted 25 Gallon Tank....Is that it?