There are 51 Event Matches to unlock and finish, but not all are immediatley playeble. here are the contents of event matches and how to get them

Event Match 1-10: Immediatley playable
11-15: Complete 6 previous event matches
16-20: Complete 10 previous event matches
21-25: Complete 16 previous event matches
26-29: complete 22 previous event matches
30: Complete 27 previous event matches
31-39: Complete all previous event matches and have Falco, Luigi, Young Link and Jigglypuff unlocked
40-50: All characters except Pichu and Mr. Game and Watch must be unlocked
51: Complete all 50 other event matches and defeat Giga bowser in Story Mode