We need one free Math teacher for every period in week

Let's take a case, when there are 5 Math teachers in your school and you wish to ensure that there always be at least one of these teachers free on every period (e.g. for substitution purposes).

You can achieve this with advanced card relationship nb.15 - "Max cards on one period" (use menu - Relations -> Advanced) with following inputs:
- max 4 cards - because there are 5 teachers and one must be always free (so 5-1=4),
- apply globally - means for all lessons of these teachers (for every class and every subject they can teach),
- change teachers - specify all these 5 Math teachers
all other settings you can leave to default.

See picture: