Betta prefer tanks that are longer than they are tall, so in a 20high I wouldn't go over 8, but as jpappy said, it's always safer to add them all together, so if you decide to add more girls, make sure you but all the new girls at the same time, and then add ALL of your girls to the new 20 gal at the same time. If someone starts to become irritable and violent, put her in a guppy breeder for a day or so, and then re-introduce her to the tank. COPIOUS amounts of foliage and plants are necessary, at every level of the tank to cut down on aggression.