Alcion requires licenses only for protected users, and not all users, in your Microsoft 365 tenant. The per-user licenses also cover backup and the required storage for all Alcion supported services, including SharePoint sites.
Protected users are Microsoft 365 users that have a backup policy which allows Alcion to create and manage backups for the user. Assigning a recurring backup policy, on-demand backup policy or performing manual backups (will automatically create an on-demand policy if there isn't one) for a user will classify them as a protected user. Conversely, assigning an excluded backup policy will remove the licensing requirement. Note that in that case the backups for the user will be deleted after a 2 week delay for security reasons.
Alcion will also classify newly created Microsoft 365 user as protected if new user auto protection is enabled.