Hi Sayyad,

Consider two Objects Account and Contact, here Account is Parent Object and Contact is Child Object.

Create a  custom field called as Number of Contacts on Account Objects to store the number of Contacts.

Then write a Trigger on Contact Object to display the count of contacts on Account Records.

Here is the Code.
 

trigger CountsContact on Contact (after insert,after update,after delete,after undelete) {
  
    set<Id> accId = new set<Id>();
    
    if(Trigger.isInsert || Trigger.isUndelete){
        for(Contact con : Trigger.new){
            accId.add(con.accountId);
        }
    
    
    List<Account> accounts = [SELECT Id,Number_of_Contacts__c FROM Account WHERE Id IN :accId];
    
    List<Contact> contacts = [SELECT Id FROM Contact WHERE AccountId IN :accId ];
    
    for(Account a: accounts ){
        
        a.Number_of_Contacts__c = contacts.size();
        
    }
    
    update accounts;
    }     
    
    if(Trigger.isDelete){
        
        for(Contact con : Trigger.old){
          accid.add(con.accountId);
        }
        
        List<Account> accounts = [SELECT Id,Number_of_Contacts__c FROM Account WHERE Id IN :accId];
    
      List<Contact> contacts = [SELECT Id FROM Contact WHERE AccountId IN :accId ];
        
        for(Account a: accounts){
            
            a.Number_of_Contacts__c = contacts.size();
        }
        
        update accounts;
        
    }
    
    
    if(Trigger.isUpdate){
        
        set<Id> oldAccId = new Set<Id>();
        
        for(Contact con : Trigger.new){
            
            if(con.AccountId != Trigger.oldMap.get(con.id).AccountId) {
                accId.add(con.AccountId); 
              OldAccId.add(Trigger.oldMap.get(con.id).AccountId); 
            }

            if(!accId.isEmpty()){
                
                // for new Accounts
                
                List<Account> newaccounts = [SELECT Id,Number_of_Contacts__c FROM Account WHERE Id IN : AccId];
                
                // For New Account Contacts
                
                List<Contact> newcontacts = [SELECT Id FROM Contact WHERE AccountId IN :AccId];
                
                
                // This is for Old Contacts Count
                
                // For Old Accounts
                
                List<Account> OldAccounts = [SELECT Id,Number_of_Contacts__c FROM Account WHERE Id IN : OldAccId];
                
                // For Old Account Contacts
                
                List<Contact> OldContacts = [SELECT Id FROM Contact WHERE AccountId IN :OldAccId];
                
                // For New Accounts
                
                for(Account a : newaccounts){
                    a.Number_of_Contacts__c = newcontacts.size();
                }
                
                Update newaccounts;
                
                
                // For Old Contacts
                
                for(Account a: oldAccounts){
                    a.Number_of_Contacts__c = oldAccounts.size(); 
                }
                
                Update oldAccounts;
                
                
            }
        }
    
    }
}

Thanks,
SFDC Beginner.



Note: If my answer helps you solve your Problem, Please Mark it as the Best Answer.