In The Flame Keepers' Circle, Ben says that he has 63 aliens. At that time, he had been known to turn into only 39 aliens, though Spitter, Buzzshock, and Articguana were in the introduction, and Omniverse would later reveal that Ben had previously used Articguana. Initial reactions to Clockwork and Eatle, and possibly also Jury Rigg, suggest that they had been unlocked prior to their debuts. Additionally, all previous aliens were reportedly unlocked by Ben 10,000, suggesting Perk Upchuck was available alongside Murk Upchuck. Ben had also been known to turn into 7 ultimate aliens at that time, which may or may not have been included in his total. At least 10 aliens are unaccounted for, and possibly upwards of 24.
In Store 23, Ben claims to have over 70 aliens. At this time, Ben was known to be able to turn into 57 aliens, counting both Upchucks. There is some evidence to suggest that Toepick may have been unlocked prior to his debut. At least 13 aliens would seem to be unaccounted for, with an upwards total of 20 or more.
In War of the Worlds pt. 2, Ben notes a plurality of new aliens. These aliens clearly were not Rath or Nanomech. The first subsequent alien known to have been accessible to Ben prior to his debut was Fasttrack, in the middle of the second season of Ultimate Alien.
I would say Fasttrack and/or Eatle might have been possible new aliens he unlocked, along with obviously Lodestar. Ben didn't seem surprised to be turning into them in Ultimate Alien, and we don't know of their whereabouts off-screen...so it could be possible he gained them in Alien Force and used them off-screen.
The migosabi wrote: I agree. He probably turned into several aliens who did not have real SUPER powers (like the worst) and that is very likely be us he has access to like 1,000 aliens and not ALL of them Can have super powers right?