Ideally, if you plan on having multiple fishes in one tank, then it would be for the best if you picked out a 15 or a 20-gallon tank. However, two female Betta fishes would do just fine in a 5-gallon tank. Female bettas are not as aggressive as male ones, especially if they are paired in a group. However, remember that they need all the space they can get, and you will still need to add caves and plants. Offering them an enriching environment is vital, especially if you have more than one Betta fish.