Super Mario 64 contains 120 Power Stars in its base game. Collecting them all is the game's main completion challenge, taking Mario through fifteen courses, secret areas, and Princess Peach's Castle.
Each of the fifteen main courses contains seven stars: six awarded for named missions and one earned by collecting 100 yellow coins in that course. The remaining 15 stars are hidden around the castle and in secret stages. Together, those totals produce 105 course stars plus 15 castle and secret stars, for 120 overall.
Mario does not need every star to finish the story. Collecting 70 stars opens the way to the final Bowser encounter by lifting the endless-stairs restriction. However, reaching 120 stars unlocks a special post-completion visit to the castle roof, where Yoshi rewards Mario with 100 extra lives and an improved triple jump.
The number 64 is a common mix-up because the game was released for the Nintendo 64, while 100 refers to the coin requirement in each course. The Nintendo DS remake, Super Mario 64 DS, expands the total to 150 stars, but that is not the base game's total.