trigger ReservationTrigger on Reservations__c (after insert) {
    // Get the IDs of related contacts
    Set<Id> contactIds = new Set<Id>();
    for(Reservations__c oneReservation:trigger.new){
        if(oneReservation.ReservationStatus__c == 'Confirmed'){
            contactIds.add(oneReservation.Contact__c);
        }
    }
    // Count the distinct Reservation_Number__c from the Reservation objects of all related contacts
    Map<Id, Integer> countDistinctReservationMap = new Map<Id, Integer>();
    for (AggregateResult aggRes : [SELECT DISTINCT_COUNT(ReservationNumber__c) resNum, Contact__c conId FROM Reservation__c WHERE Contact__c IN: contactIds GROUP BY Contact__c ]) {
         Id conId = (Id) aggRes.get('conId');
         Integer resNum  = (Integer) aggRes.get('resNum');
         countDistinctReservationMap.put(conId, resNum);
    }
    // Now fetch the Contacts in a list
    List<Contact> listContacts = [Select Id, customRollupField__c from Contact Where Id IN:contactIds];
    if(listContacts.size()>0) {
         for(Contact con : listContacts) {
              // fetch or get the distinct count or rollup from the map and copy it to the contact's field
              con.customRollupField__c = countDistinctReservationMap.get(con.Id);
         }
    }
    // Update the contacts
    update listContacts;
}