The number of spots to make available is an important consideration for a contest organizer. Here you a determining the number of entries you will permit in this contest.
Are you looking for a large contest with many entries and a large prize pool or a smaller intimate contest among a small group of writers?
When deciding this keep in mind the voters. When the contest deadline is reached all entries will be placed in a voting booth. Larger voting booths place a larger burden on the voters.