No one can say for sure. In one of the episodes of the original series, Ben and Gwen are taken to the future, and there you find out that Ben 10 has become Ben 10,000. Furthermore, in an episode of Ben 10 Alien Force, the second to last episode of the second season (Season 2, episode 13), the creator of the Omnitrix activates the master control, so Ben asks, "How many aliens can I turn into?" To which the Omnitrix replies, "One million nine hundred three." That's 1,000,903. So basically, no one will ever know what all the aliens are. However, for the most complete list available, see here: http://en.wikipedia.org/wiki/List_of_Omnitrix_aliens