A user is defined as an employee or a supplier who has or will have access to Odoo in create and/or edit mode. External users (such as clients or suppliers who are portal users) are not counted in the number of users. For example:
An employee who reports their expenses or timesheet is counted as a user;
A third party accountant who uses Odoo to work on your numbers will be counted as a user;
Customers and suppliers that have access to their invoices on-line will NOT be counted as users;
Visitors to the Odoo website will NOT be counted as users.