User blog comment:R E W S/The Best champion in the League of Legends:Arcturus, The Stellar Morpher (FINISHED)/@comment-1694864-20121203154621/@comment-99.10.120.85-20121203195732

10 combinations if order doesn't matter, 27 if it does. The original design for Invoker had 27 spells, but this was nixed as keeping track of the order of your QWE constantly was less fun than you might think.

Personally, I think the 12 forms is more like a flavor thing (12 Zodiac signs) than something that's been actually worked out, since arbitrarily mixing combinations and permutations is a baaaaad idea from a user standpoint.