You can also limit the maximum number per group manually in the Pro version. In order to do so, please go to:
Overview => Settings => Manage Participant Groups
and click on "Edit" to edit the corresponding participant group.
On the following page, untick the check-box for "Group Active and Visible?" in order to deactivate this participant group.
It then will now not be shown anymore to users or registrants who wish to register for participation.