From the Main Menu bar, click on File / Entry/Scoring Preferences / Entry Limits.
Entry Limits for the Entire Meet
Customize your meet entry limits (if any) for Individual and Relay events. For example, you might want to restrict each Athlete to a maximum of 4 entries including relays with a maximum of 2 field events and 2 track events including relays.
Warn if Entry Limits Exceeded
Click on the check box and MM will give you a warning when Entry limits are exceeded and give you an opportunity to override the limit.