Is it possible to only have just one rat living in my house?

We moved into this house 2 weeks ago, and while we were moving in we noticed some droppings out in the garage. This is a single story home on a concrete slab with an attic. From time to time since moving in, I would also hear a little rustling in the wall that is the garage on the other side. Well, last night I finally saw the stupid little thing when I went out there to get something. And then tonight he was much more social and didn't try to hide. He is the only one I have seen, it appears the garage is it's hangout and there is no evidence (knock on wood) of it being inside the house, just the garage. So, is it possible that he is the only one? Or could there be more?