When you reach your subscription limit for the number of users/staff within your account, you'll get this error message below:

Number of users can not be greater than max subscription limit.

In order, to fix this issue please get in contact with your Account Manager who can help increase your staff/client subscription limit. 

If your role is Admin, you can adjust the number of licenses available by following the steps in the article: Increase the number of staff licenses - upgrade subscription