You don’t need to buy a license for every team member. Once license is enough to back up your team's data.
However, due to security & privacy reasons we can only back up the data of the users that have authorized Pro Backup: Private projects or boards from other team members are not included in the backup.
If you wish to ensure that all data is backed up, you can either:
Invite every team member to Pro Backup (via Settings > Connections)
Ask your team members to add the account of the admin to each project or board (then this data will be added to the backup in the next cycle)
Tip: It can be useful to create a dedicated account to manage your data backups (e.g. [email protected]) and ask your team to always invite this user to their projects and boards.