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The Count Field is unable to Count the collaborator field. To return a count (length) of users from the Collaborator field, Base administrators are forced to employ a Formula that is not ideal.
A base administrator can add a count field to count their users within a Collaboration field without having to Google Search why it's not possible, only to find a formula hack is needed.
Everyone who expects the Count Field to be useful with counting various other elements that are found within the many other Airtable Field types - such as, but not limited to, the User collaboration field.