In the original release of Super Mario 64, there are 120 - this number was upped to 150 in the DS remake. In Galaxy, you can collect 121 in total, but the 121st star can only be obtained by playing through the game 100% as both Mario and Luigi, which leads to a secret mission after the final boss. Galaxy 2, meanwhile, has a whopping 242 stars in the game, counting both regular and Green Power Stars.
Also, they're not exactly Power Stars, but there are a total of 120 Shine Sprites hidden throughout Isle Delfino in Sunshine - they serve the same purpose, collection-wise. 07:23, February 16, 2016 (UTC)