Thank you for your answer but I'm sorry to not understand what you mean by "ofc you can't use e.g. list field or similar". The field I try to sort is a "SQL request" type:
I have a list of "things" - each activity has a certain amount of "features". The SQL field counts how many "feature" each "thing" has and returns the result as a number in the list of "things". This works fine.
I can order (asc or desc) this list by the name of "things" but ordering by the amount of "features" does not work, whether I set the type "text" or "numerical".