There are no restrictions on the amount of users you can add to your Odoo instance. The actual number of users depends on your server’s resources and customizations. For example, an Odoo instance without customizations on a server with 1vCPU and 1GB RAM can accommodate up to 25 users.
However, if you have an Enterprise license you will still need to pay a license for each user. If you opt for the open-source Community Edition of Odoo, there is no extra charge per user.