The mouse rack will be fine as a maternity rack if you choose to use one.
And as to how many females you need. The general rule of thumb is 1 female per snake give or take. You may have to add more females depending on how often you feed. I only have 9 snakes, but have about 15 breeder female rats. I don't harem breed any more. I manually place the male with certain females (2 females every 2 weeks). And I've found a really nice schedule with the every 2 weeks thing. It gives me 2 litters every 2 weeks and it's about 2-3 weeks worth of food. The really fancy ones (dumbos, rex, dumbo rexes, dumbo double rex) get sold as pets. And any other extras get fed to the ferrets.
This sticky was pretty helpful when I was figuring how many females to have. http://ball-pythons.net/forums/showt...-Many-to-Breed