You subtract festivals by a list of inschrijven or the other way around.  That's like subtracting appels from pears.

Not sure what you want to achieve, but this will never work😉

//Edit

As I understand you want to check if there are still spots left for a new reservation. First I assume that the user selects a festival. Thus we can use this festival as context of the process.

Your microflow should be like this

1. Start with an input parameter > Object > Festival
2. First activity > retrieve all reservations (Inschrijven) over association of that festival 
   If you don't have an association between the Inschrijven and Festival entity, you should create one.
3. Add a 2^nd activity > Aggregate > Count > returned list of 1^st activity
4. Add a decision > check Festival/maxAmount > ReturnedCountValue
5. True continue with new registration (inschrijven)
6. False Show a message to user and end microflow

Domain model

Microflow