The answer, of course, is forty-two, which is either six times seven (base ten), or, as the cavemen found with their scrabble letters, six times nine (base thirteen). I have enough trouble coping with one planet (bigger than my brain), let alone multiple universes.

By definition, we are only in one universe (uni- meaning 'one').  This is all of space and its contents. 

There are hypotheses that speculate that the universe is one part of a larger multiverse. 

There are still others that suggest the universe is the intersection of two brane-verses, which are multidimensional realms of existence.  This latter idea has the universe being the intersection of  two colliding branes, while everything outside the universe belongs to one braneverse or the other.  These branes are higher-dimensional (last I heard, they needed 11-dimensions to work) and are only conceivable in abstract or mathematical terms.  In this idea, the Big Bang was the moment the two branes collided, and the expansion of the universe is analogous to the area of intersection increasing.  Once the branes pass through each other, the universe will no longer exist (no more intersection).

There is also an idea that the universe is holographic - essentially that all the stuff inside the universe is just a projection from stuff happening at the universe's boundary.  This could allow higher dimensional projections to be other 'universes' within, but distinct from, our own.  There could also be projections in other directions that form other 'universes'.

Finding out more about the true nature of our universe should help solve some of the outstanding mysteries of the universe, possibly even leading to the GUT.  Still a long way to go, though...

Well, what indeed? David Deutsch's Book "The Fabric of Reality" (I think that's right, but my copy seems to have gone walkabout between the dimensions) tells you more about that than you presumably meant to ask. He argues impressively, but I could wish he sounded less confident. The concept of "boundary" is very tricky in these matters. What is the boundary of a Klein bottle? What is on the other side? How many dimensions are involved in the bounding? What do you mean by "Where" in this sense? Next to a particular galaxy or something? If "universes" "intersect" in what sense can they be universes without each other? I sense a need for you to check your semantics, the meaning of your words, before you apply them to "objects" to which those senses might not apply. By way of analogy, what would you say to anyone who says: there are North and South poles, so there must be East and West poles, and also NE and SW poles? And where are they? It sounds good till you begin to examine the meanings carefully, then it all goes pear-shaped, and where are your poles and for that matter, Rumanians then? Cheers, Jon

I'm not able to discuss this topic using the tools of mathematics and the latest results of astrophysics, even if I have a Master of Science level University degree in Technical Physics - from long time ago. To me the idea of multiple universes sounds good if we base our thinking on infinity. I have the kind of thoughts found below. It probably is so that many others have been thinking something like this also, including true scientist in their scientific work, it would be nice to learn more about the topic.

There is an infinite number of infinite universes in an infinite space. We know that there can be two "sizes" of infinity (all integers, all real numbers) as discussed in Cantor's continuum hypothesis.

As a special case, one of the universes is totally empty.

The other universes have an infinite number of possibilities for anything at all to exist.

Time is infinite both towards past and towards future.

We can pick any time (past, now, future) and find an infinite number of universes in a random set of states.

There was not just one Big Bang, but Big Bangs are happening all the time, due to collisions of universes. What a collision exactly would mean I have no tools to describe, I just imagine a collision.

The broken symmetry is explained by random collisions with matter of different universes in random states.

Dark matter and dark energy (and the Pioneer anomaly) is explained by gravity of other universes.

The accelerating expansion of a universe is explained by gravity of other universes.

Since there is an infinite way of arrangements in a universe, there is an infinite amount of life forms.

Even if life forms are infinite, there are a lot more arrangements with no life.

Locally (by time and space) it is unlikely to find life in more than just one place. This is why we have not so far found extra-terrestrial life.