There are 15 courses, each with seven stars = 105 stars
Then you get an extra star in each course if you get 100 coins in that course = 120 stars
Finally, there are 30 castle secret stars dotted around the castle = 150 stars
A lot of these are very hard to get, and you will need to beat bowser and other bosses
along the way, so good luck!
Fyi, you will also need to assemble all four heroes, but these are easy to get.