How many universes are there

Any time we seem to grasp something about nature, something else turns up.

We have 100 billion stars in a galaxy
We have 100 billion galaxies in a universe

Could it be that there are 100 billion universes?

There are clearly a lot of people with very fixed views on the subject who don't have access to various papers etc published in the physics community. However, the stupidest answer of all is the one that says because the word Universe means "everything there is" there cannot be others. The time may well have come to change the meaning of the word to mean "our bit of everything there is". Contrary to popular belief, hard evidence is beginning to stack up suggesting the existence of other universes, though the findings are fairly esoteric and by no means complete. Those who decry M Theory and it's relatives might like to think of it in a slightly different way. Perhaps they might be able able to accept that "the Universe" is very, very much larger than was supposed, and split into different regions, each with it's own properties. They could then carry on with thinking that "universe" means all that there is while the rest of us get on with wondering about the properties of those other "regions", which we will continue to call "other universes". Unfortunately, quite a large proportion of papers published are not readily available to those outside of academia and so much of the information stays within the physics community. This is, I suppose, understandable, because unlike populist publications, much of the work is mathematical in nature and not readily understood by non-specialists. However, to answer your question directly, there may well be 100 billion other universes (or if you prefer, regions) but we have no real way of knowing. There could be one other (unlikely) or an almost infinite number.

Multiple universes have been hypothesized in cosmology, physics, astronomy and even in philosophy. The various universes within the multiverse are sometimes called parallel universes. One aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations, each with a different probability. Each of these possible observations, might as well, corresponds to a different universe.