Let me see if I can back you into an answer.
If I recall correctly -- and reading between the lines of your post -- usually the limitation is placed on the Contributor license, and when the limit is exceeded, Workfront will automatically upgrade the user to a Light license.
I'm also wondering if this only applies to proof and document approvals for you.
If so, COULD this be one or two reports -- one for doc approvals, and one for proof approvals, grouped by month (or filtered by approvals this month), where the approver has a contributor license?
Similar to the screenshot below.
