Not at all. You request a total number of users at a domain and then you can delete and create accounts to stay within that user account limit. This is useful for companies with high employee turnover rates such as interns and temps as you will not have to purchase additional accounts to replace these users.

With the old employee's data, Google will purge it when deleted. Click here to learn more. Keep in mind that you can't change the account's username, however you can add nicknames to handle the revolving door of employees.