I know that there should be more, but let's say that there are seven million humans on the planet at the time you are reading this and that no one will die or be born until you finish. Let us further assume that, despite the vast multitude of the universe, that there is no other sentient life other than what is on our planet. I would then contend that there are seven billion and one universes in existence.
Now, your first question would be to ask what I was smoking. Those of you who know me personally know that I don't smoke anything. Other may assume that I'm hit my head too many times and I've gone around the bend. I'm not sure I can argue against that (would I know if I was crazy?), but I think I'm sane enough.
But how, Mr. Matt, can there be more than one universe? I'm glad you asked that, Timmy. I would contend that there is one objective universe that we all share and one subjective universe for each of us. While some philosophers may argue that the universe is either subjective or objective, but not both, or that I'm not counting the infinite parallel universes that may exist. Well, those philosophers need to take a deep breath and get back to delivering pizzas or flipping burgers. I may talk about the parallel universe theory one day, but not today.
There is a single objective universe that we all inhabit. It would be here whether humanity was or was not. I will be here no matter how many humans or sentient being exist. But for each being, there exists inside our heads a subjective, or perceived, universe. I perceive the universe in a way no other being ever has or ever will. Each and every one of us is a different genetic and neurological self. Even twins are different in that they must perceive things differently and therefore their brains will be slightly different (and become more different over time).
Ever hear (or pretend to hear) something no one else does? It's real to you. Ever see a ghost (whether it was real or not)? It's real to you. Ever eat the wrong kind of mushroom and start smelling colors? It's real to you. Each of these is part of your subjective universe. Your subjective universe is how you perceive the objective universe.
Does that mean that every subjective universe is equal? Of course not. Charles Manson's subjective universe leaves a lot to be desired. Martin Luther King Jr's subjective universe is a far better one. You, with your subjective universe, may disagree with one or both of those statements.
Of the two types of universes, only the objective universe is consistent and understandable to all. It is what allows us to interact with each other, but only as long as our subjective realities mesh well with the objective reality. If two subjective realities diverge too much, you get long term issues like Israel and Iran, Democrats and Republicans, or Wile E. and the roadrunner.
The next time you find someone you don't understand, just think about what their reality may be like, and try walking a mile in their universe. You may just learn something.