Is the same Trello user a member of 2 or more boards in your Workspace? If so, they are considered a multi-board guest and are billable. This page has more info: https://support.atlassian.com/trello/docs/multi-board-guests/
If this doesn't answer your question, could you share a screenshot of where you're seeing this?
Thank you for the information. I had Trello a long time ago. If I delete the old boards, will I be billed for just the one board?
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If you are the only member on the remaining boards, you should only be charged for the one member (yourself).
The page I linked above shows you how to remove guest users from your boards. You can do that instead of deleting the boards - whichever works better in your case.
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