If you're an Enterprise admin, the user management dashboard will include a box of helpful statistics about your license count at the end of your left-side menu.
The Members count in the left-hand sidebar (highlighted in gray) includes a list of all billable Workspace members (licenses), outside collaborators, deactivated members, and Trello members with SAML credentials but no Workspace memberships.
The bottom stats box (in blue) includes a count of:
Billable members: active members of your upgraded Workspaces. These users are considered to take up a license seat.
Unused license count: unused licenses for your Workspace (optional).
Outside collaborators: members of your boards who aren't members of any upgraded Workspace. These users don't take up a license seat.
Deactivated members: deactivated members of your enterprise Workspaces.
For users with one enforced and one unenforced enterprise, these stats are per-enterprise, rather than per-billing-unit. As a result, the stats will not deduplicate users between enterprises, and adding up total billable member count will result in an artificially high count. If you are in this situation, please email your Enterprise Account Manager for an accurate license count.