There can be two Math lessons on one day, but they need to be placed consecutively

If you have 6 Math lessons per a 5-day week, then two of them will fall on the same day. In this case, you can tell the software to place these lessons consecutively.

Add a new card relationship and select the entries marked in red:

The entries marked in green are optional. For example, if you have 6 Math lessons, the software will by default spread them across 5 days (one of the days will have two lessons).

However, you can instruct the software to place all 6 lessons within 4 days. Or, as shown in the picture, you can let the software decide whether to place 6 Math lessons over 4 or 5 days.