Two would be better so they can keep each other entertained, but you said you wanted an adult cat and I'm sure there's some poor elderly kitties who would be fine being an only pet. Sometimes the older rescues weren't socialized and don't get along with other animals, so that would be totally fine for you. If that's the case, you almost certainly would have to separate the cat and dog.

I have a "problem" cat that was a spoiled only child, though she begrudgingly accepted a new sister after a while. I'm convinced she would be absolutely content as an only cat.