I have 4. 75 and 20L are my main tanks. Also have a 20H that was being used for quarantine. My 10 is dry right now. I'm thinking of making it my new quarantine tank, because the 20H needs to be resealed.
dbcb314, it also amazes me how people manage to keep so many aquariums. With filter maintenance it must be crazy. The most I could probably manage is one more aquarium. Wish I could keep more, but I would have a hard time unless I made it my full-time job(owning LFS or being a breeder)