How to count and display number of people by date?

Question

I want to count the number of users on each day and display a different icon on the calendar for each number of users. How do I implement this?

thank you very much if you can solve it

firebaseDatabase.getReference().child("calendar").addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        for (DataSnapshot snapshot : dataSnapshot.getChildren()){
            if(dataSnapshot.getChildrenCount() == 1){
                String key = snapshot.getKey();
                int[] date = splitDate(key);
                Calendar event_calendar = Calendar.getInstance();
                event_calendar.set(date[0], date[1], date[2]);
                EventDay event1 = new EventDay(event_calendar);
                events.clear();
                calendarView.setEvents(events);
                EventDay event = new EventDay(event_calendar,R.drawable.gyeong);
                events.add(event);
                calendarView.setEvents(events);
            }
            else if(dataSnapshot.getChildrenCount() == 2){
                String key = snapshot.getKey();
                int[] date = splitDate(key);
                Calendar event_calendar = Calendar.getInstance();
                event_calendar.set(date[0], date[1], date[2]);
                EventDay event1 = new EventDay(event_calendar);
                events.clear();
                calendarView.setEvents(events);
                EventDay event = new EventDay(event_calendar,R.drawable.ic_baseline_boy_24);
                events.add(event);
                calendarView.setEvents(events);
            }
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError error) {
    }
});