There are no restrictions on creating free users. You can create any number of free users
Can a free user use mail?
A free user does not have the ability to view email. The message will be displayed as a note. He can not disclose the contents of the letter. Even when you open the "lock" on a personal letter, it will also be unavailable.
Free user and tasks
A free user can not work with the task functionality, including he has no rights and to view other people's tasks. Any other user can not put a task on a free user.