I currently have three bettas, but I'm caring for my sister's betta too, so I guess you can say I have four. I have one divided 10 gallon which houses my male, Patriot, and female betta, Tiny Tina, and two zebra nerite snails, which have made babies that will never hatch. :( Poor little things. I also have a 3 gallon KK which houses my baby betta, Norman. My other 3 gallon KK houses my sister's ill betta, but he'll soon be going back to his own tank.
The only thing preventing me from buying more bettas is that I don't have any more available heaters. Plenty of tanks, but no heaters. :( And my college dorms only allow me to bring one 10 gallon tank. I'll be dividing my 10 gallon into three parts so once my baby is big enough, he can go in it too with his own nerite snail, most likely.