The number is equal to the number of days on your base ticket (or two admissions for one-day tickets), though they can be used anytime. If you have seven-day tickets, it costs just $10 per ticket per day to add this option; the value of being able to visit one park in the morning and another in the evening should not be underestimated. Without it, you could be stuck with nowhere else to go after doing everything you want to do at one park; or you could run out of time to do one last attraction and be unable to come back another day to pick it up.