This issue has been raised in the past (e.g. https://moodle.org/mod/forum/discuss.php?d=269333 from 2014!) and I find myself and my colleagues still confronted with it every year.
The problem is that when I want a schedule in groups, and my groups have for example up to 5 students, and a group with only 3 students makes a booking, the scheduler allows to book the same slot with a group of 2 students.
Our scenario is that we make an oral exam for group of students and we can handle only one group at a time.
The solution would be to restrict the "maximum number of groups" and not "maximum number of students".
I understand from the thread from 2014 (see link above) that this was considered for implementation, but as far as I can tell this never made it into the plugin.