All contests in a league need to fill up in order to run, unless you have set the contest to be resizable.
If a contest is resizable and does not reach the initially intended number of entries, it will be automatically resized to the current number of entries it has. This only applies to contests with less than 20 entries. Here you can learn more about the resizing rules.
If a contest is non-resizable and does not fill before the start time, all entry fees will be refunded.