All standalone maps should have a unique map number. This is required so there are no conflicts with other maps.
This is done because of the old game engine working with numbers and it allows players to track their completion of maps in-game.
You can use this tool to find an available map number for you to use so it can be uploaded to the KeeperFX workshop without any conflicts.
You can also find a full list of used map numbers here:
https://keeperfx.net/workshop/map_number/list/map