That's good for whatever action figure company has the Ben 10 license, but daunting for a game designer to tackle – so, wisely, this first Ben 10 game chooses to focus on only a subset of Ben's best transformations. But as the series has progressed, he's gained more and more accessible alien forms to call upon, with new ones added with regular frequency.
The titular numeral refers to his original set of 10 transformations – the Omnitrix on his arm, when activated, allowed him to assume the shape, abilities and psychological mentality of any of 10 different alien species. Ben's dealing instead with upwards of 20 different personalities and alternate alien forms in the cartoon show, and it's kind of a mess to try to keep track of them all. Because Ben's double life isn't just a double.
