WebAdmin supports a maximum number of accounts that can be created on a per-domain basis but this option is actually less visible then you might imagine.
To set the maximum number of accounts, login to WebAdmin, go to the “Domains” editor, edit a domain and look at the [Properties] tab and look for the “Maximum number of users that can be created by WebAdmin:” option.
This limit effects domain administrators, but not global administrators.
Note that this option is not exposed in MDaemon’s GUI anywhere, so in order to take advantage of this feature you need to be using WebAdmin.