I have 12 now. All are on FT except one who is sometimes on FT and sometimes not. She's had RI issues and going back to the vet, so we'll see if I can get her sorted out. When she's doing well she takes FT no problem along with the others.
If I had a lot of snakes, I might setup a rat colony to breed my own live feeders. With just 12, though, FT is convenient.