How many mistakes? I count three, which, in my opinion, is three too many for this published author.
Attendance in our 12 week course gives you access to special pricing (up to 50% off) on noted live events. (Note: not all live events will hold the same discounts, please check with us for details).
My edit:
Attendance in our 12 [where’s the hyphen?] week course gives you access to special pricing (up to 50% off) on noted live events. (Note: not all live events will hold the same discounts, [comma splice; need a semicolon] please check with us for details). [Why’s the period outside the parenthesis?]