If we do, in fact, live in a multiverse, with multiple universes arising out of the Big Bang, how many are there? Andrei Linde and Vitaly Vanchurin at Stanford University have been working to calculate a number, based on quantum fluctuations in the early state of the universe. Their tally indicates that there are at least 10^10^10^7 universes out there. The human brain, however, could not possibly distinguish between all of those universes, however, as it is only capable of 10^10^16 configurations.

How many universes are in the multiverse? [arxiv.org via Universe Today]