Collaborators are nothing but your Team members that you add in your Team tab. There is no limit on the number of team member you can add to your team.

To add your team, you need to go to the Team module and click on Invite button. First add their email Id and then choose to add them as member or as admin. Admins will have access to all modules, while for members you can decide which module to give access as shown in access settings.

To add a user as a collaborator to a gallery or a project, that user needs to have access to the that module. After adding as a collaborator, that user will be able to make all changes within that gallery or project, except deleting it. Only admins have the permission to delete any file from a module.

To know more about how to add team members and assign them as collaborators, you can learn how to manage your team.