Average generally around two unfortunately, especially with unspayed ladies. Mammary tumors are super common :/
I've had many rats over the years, and I'd definitely would love to have some again. They're very social animals, so at minimum you need a pair. They're very active and curious, so they need very large cages (wire--no tanks) and lots of things to keep them entertained (you can always go the toilet paper tubes, cardboard boxes, etc route to help make it cheaper). Plus out time to explore. You can pretty easily "rat proof" a bathroom for this.
They're really prone to respiratory infections, and vet care can add up for them. If you can swing it, I'd definitely recommend getting them spayed. Mammary tumors get huge on these guys compared to there body size. Like golf ball size or bigger.
They're great pets, but they're going to be much more involved than a reptile so keep that in mind.