When an employee leaves the company, it is wise to inactivate them.

This ensures that a salary is not calculated inadvertently for someone who has left. Putting an exit date does not automatically inactivate someone since it is possible to calculate salaries out of period (see here)

On the other hand, licenses are based on the number of active employees in the company.

Go to Employee management : 

The button works both ways: it can be used to inactivate someone or to reactivate him/her if he/she has been inactive. When you click on it, the tab with the entry and exit dates opens. If everything is ok, just click on OK.

The employee management screen and the calculation screen have a checkbox to display only active employees. This can be useful if there are many employees.

Note that an employee cannot be deleted purely and simply if he has had a salary during the year.

At the end of the year, the inactive employees will not be checked and you will have to check them if you want to carry them over anyway.