My requirement is incremnet of "No_of_JobApps__c" when i inserted/Undeleted Job_Application__c related to a Position__c and decrement of No_of_JobApps__c when i deleted Job_Application__c related to a Position__c...
1. No_of_JobApps__c is a number data type on Position__c custom object
2.Job_Application__c is related with Position__c(Master object). code is fine but i am not getting No_of_JobApps__c field updated.
Trigger code:
trigger countJobApp on Job_Application__c (after insert, after delete, after undelete) {
    if(trigger.isAfter){
    set<id> jobAppIds= new set<id>();
        if(trigger.isInsert || trigger.isUnDelete){
            for(Job_Application__c jobs:trigger.new){
                jobAppIds.add(jobs.Position__r.id);  
            }
        }
        if(trigger.isDelete){
            for(Job_Application__c jobs:trigger.old){
                jobAppIds.add(jobs.Position__r.id);
            }
            
        }
    list<Position__c> pos= new list<Position__c>();
    list<Position__c> jobsforPos=[SELECT No_of_JobApps__c, id, (SELECT id FROM Job_Applications__r) FROM Position__c where ID IN :jobAppIds];
        for(Position__c jobPos:jobsforPos){
            integer count=0;
            for(Job_Application__c japps:jobPos.Job_Applications__r){
                count++;
            }
            if(jobPos.No_of_JobApps__c !=count){
                jobPos.No_of_JobApps__c = count;
                pos.add(jobPos);
            }
        }
        if(pos.size()>0){
            update pos;
        }
    }
}