I originally had a cage that was about 25"x15"x15" that I'd gotten for free. I had my first few rats in there, but it wouldn't house more than two females comfortably. So three years ago I got my R-690 for my birthday, and it has been terrific! I also have a smaller cage, a bit smaller than that original cage, that I use for quarantine and travel. Well, I have a carrier for travel, but the smaller cage is for staying overnight places, etc - since dragging the R690 in and out of motels is not fun!
Unfortunately, my R-690 is getting a bit worn. The powder coating is chipping off here and there, and it's got that nasty white buildup all over. I've tried soaking it in vinegar water, I think it only made it worse! So within the next year I'd really like to get a new Martin's, although for now it's still holding up fine. If much more coating wears off, though, it'll start smelling.
So if you count my carrier (which really isn't a cage, it's like 15"x12"x9"), I've had four cages, three of which I still have. I've never had more than 5 rats at a time, I currently have 4.