I do not think that Andrej is right in this case. Currently Mendix does limit the amount of active accounts depending on your license. So what you should do is create an object which holds your value with the current max active user accounts. Then you can retrieve all active account to check how far your are in consuming them all.
AFAIK mendix only limits the number of accounts that are accessing your app at the same time. (called internal users in the new pricing model)
There is no limit on the number of accounts that you can have in the database.