Hello Ingo,
If I understand you correctly you want to count the number of records within a specfic time range and linked to a specific project, you can do a set up like this within the microflow, where you retrieve your records based on your xpath and then count the list.
Benefit is that Mendix optimizes this action when you do not use the specific list later in your flow mendix just gives back the count :-) .
More info can be found here:
https://docs.mendix.com/refguide/aggregate-list/