Wow I'm a total cat novice. I've only had two at the same time, and that didn't work out nearly as well as with just one. The older cat (in terms of how long he'd been with us, not actual age) immediately set to work proving he was alpha. They never fought seriously and no one was ever hurt, but I felt bad for the new cat, because he just had a sweeter disposition. He'd want to sleep with me on my bed, but the older cat wouldn't let him and would sit on the bed and glare at him until he left. When I moved, I left the new cat with my (now ex)boyfriend, and he told me it was like he became a whole other cat, who was more confident and friendly.
As long as I have my grumpy cat (with resource-guarding issues, I think), I'm probably going to stick to just one cat. So much for my dream to be a crazy cat lady.