Hello, Antonella!
Welcome to the community.
Trello is charged based on the number of users that are in your boards, I believe it's possible you have a user on this workspace that is on two boards or more, this is why is charging you two licenses.
Every user will be checked individually on Trello Paid workspaces to test if they're billable. It means, that every user will be billed if they fit billing rules. However, some users aren't billable on Paid Plans, like single board guests, I'll clarify this:
Workspace Members:
These users aren't necessarily Board members; they can see all the Boards in Workspace, create new Boards, etc.
All the Workspace Members are billed.
Guests:
Guests are users who aren't Workspace Members; however, they are members of one or more Boards on Workspace. They aren't able to create Boards, see other Boards in the Workspace, access custom Views, etc. They need to be more tricky regarding the billing question.
Single-Board Guests: Members of one Board of the Workspace (regular users or observers) aren't billed.
Multi-Board Guests: Members who are members of two or more Boards of the Workspace and are billed.
More information about this topic can be found in the articles:
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