As a General statement, there will be different sets of ruels for each contest.
In the rules, the contest must have a set amount of things before being initiated.
A limit of entries (if any).
A starting time.
A stopping time.
What's accepted.
What's not accepted.
How many winners there'll be.
How the winners are picked.
How many judges, and who they are-
Or when the voting period will begin.
If in the case of judges, those judges, out of fairness, will not be allowed to enter the contest. If it's done by a vote, then anyone can enter as long as the rules are followed.