Can I define min/max supervisions for the teachers?

Yes, in case you need, you can define the limits for supervision for certain teachers. It can be set in teacher's constraints dialog and you can limit the count and/or total minutes of supervision.
If you do not want to limit teacher, just leave the fields blank.

This feature is usefull, if you use the duties generator, but even if you are assigning teachers for supervisions manually, you can see these limits in the list of teachers (only maximum values).

You can also check the actual values along with min-max in the menu TimeTable - Statistics :

Note:
The "count" of supervisions has higher priority than "summary lenght of supervisions" (in minutes). It means, that max count of supervisions will be kept, but summary duration can be slighty different (it depends on your bells settings).