A multiverse is a set of universes related to each other. You can punch through them to communicate or travel to others, so Kate could go to your world, but she couldn't go to Sam's world because it's in a different multiverse. But there might be other universes in his multiverse where he's different. It's like books in a library, you can read all the different books that are there, but you can't read the books that are in a different library.
Then there's realms like Limbo, which are worlds of their own that are attached to a single universe inside a multiverse. And parallels, which are outside a multiverse, but sort of... attached to it. Normally you can't get back and forth between them even though they're related to each other. There could be multiple versions of someone from a realm if there are other versions of that realm in other universes, but you don't get multiple versions of people from a parallel. I guess you might here though. Weird rules here.