As far as current knowledge from science is concerned, there is only one universe, the one we see when we look around earth and look into the sky. However, a number of prominent scientists believe there may be multiple universes or multiverses.
The following article from Wikipedia gives an introduction to Multiverses, including arguments for and against.
http://en.wikipedia.org/wiki/Multiverse
Support Tree for "Agreement" Camp
Total Support for This Camp (including sub-camps):