Try using count on the list aantalLunchesMedewerker_1, this should give the count you are looking for. The create Lunch_InformatieList is redundant.
In addition to my previous answer I also want to suggest a different approach to the problem above. Create a domain model like the screenshot bellow. On the LunchInformation_NewEdit you can put a Listview and use the association to LunchParticipant as Datasource (make it editable!). Finally, put a reference selector on the Listview using the LunchParticipant_Employee as source. You will need some custom buttons. For instance, on the LunchInformation you will need a custom New button that triggers a microflow that creates both a new LunchInformation-object and a new LunchParticipant-object (set the association to the LunchInformation-object). If you commit these objects using a custom Save button you will be able to Retrieve a list of employees over x-path and count the retrieved list. Hope this will help a bit.
In this Mendix-video there is an example of how to do a count, maybe this will help you: