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 doesn't reach the initially intended number of entries, it'll be automatically resized to the current number of entries it has. This only applies to contests with fewer than 200 entries.

Important Note: A contest cannot be changed to resizable after it has been created. 

If a contest is not set to resizable and does not fill before lock, it will be canceled and all entry fees will be refunded.

Learn more about the resizing rules.