What does the term collaborator mean?
This is the total number of unique Tracker users that can be member(s) of the private projects in your account with read/write project access. This includes yourself, if you are a member of any project in the account. A given user only counts once, regardless of the number of private projects in your account that he or she is a member of. Read only members (viewers), and members of public projects do not count toward collaborator limits.