Hi Subodh Kumar,

We can not use rollup in lookup relationship. If you need roll-up functionality in lookup. 

Create a field to show the rollup field value.

Here is the sample code

Here I have created a field "Total_Contact__c" in the account, to show total num of related contacts.

trigger accountTrigger in Account(Before Update) {
    set<Id> accountIds = new set<Id>();
    Map<Id, Account> accountMap = new Map<Id, Account>();
    for (Account acc : Trigger.New) {
        accountIds.add(acc.Id);
        accountMap.put(acc.Id, acc);
    }
    List<Contact> contacts = [SELECT Id, AccountId FROM Contact WHERE AccountId IN: accountIds];
    List<Account> toupdate = new List<Account>();
    for (Id ids : accountIds) {
        for (Contact Con : contacts) {
        integer count = 0;
            if (Con.AccountId == ids) {
                count++;
            }
        }
        //Here you can put the field name on which you need to update total number of contacts
        accountMap.get(ids).Total_Contact__c = count,
        toupdate.add(accountMap.get(ids));
    }
    Update toupdate;
}

Please try it.

I hope it helps you.

Kindly let me know if it helps you and please mark as Best Answer.

Thanks and Regards,
Biswojeet