Solutions
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Bl4ck_iris's post in How many hidden Mickeys to unlock the secret ending? was marked as the answerI'm pretty sure it's:
30 on Proud
60 on Standard
90 on Beginner
source: https://www.gamerevolution.com/guides/490307-kingdom-hearts-3-secret-ending-movie-unlock-lucky-emblem-beginner-standard-proud