Looks like you'd need to either:
a) replace the filters with relationships to multiple occurrences
b) use ExecuteSQL to count them
c) set up a table with one record per day that matches to the records shown in your calendar and then you can define a count function in this record that counts all records for that one day.
I'm not sure if this will help in your context but I have created a demo file with an Calculation field with the Execute SQL function to count filtered portal records as Phil mentioned above.
Tables are 'Parent' and 'Child'
Parent Layout has 2 portals - Unfiltered and filtered and 2 calculation fields one of which counts all related child records (the unfiltered set) and the other uses the Execute SQL function to count only those that match the filtered criteria.
You can change the filter to see the calculation working.