Create an Apex class with a method using the @future annotation that accepts a List of Account IDs and updates a custom field on the Account object with the number of contacts associated to the Account. Write unit tests that achieve 100% code coverage for the class.
Create a field on the Account object called 'Number_of_Contacts__c' of type Number. This field will hold the total number of Contacts for the Account.
Create an Apex class called 'AccountProcessor' that contains a 'countContacts' method that accepts a List of Account IDs. This method must use the @future annotation.
For each Account ID passed to the method, count the number of Contact records associated to it and update the 'Number_of_Contacts__c' field with this value.
Create an Apex test class called 'AccountProcessorTest'.
The unit tests must cover all lines of code included in the AccountProcessor class, resulting in 100% code coverage.
Run your test class at least once (via 'Run All' tests the Developer Console) before attempting to verify this challenge.

Illegal assignment from Integer to String when trying to save

code that I'm trying to use line 11 is the issue

public class AccountProcessor
{
  @future
  public static void countContacts(Set<id> setId)
  {
      List<Account> lstAccount = [select id,Number_of_Contacts__c , (select id from contacts ) from account where id in :setId ];
      for( Account acc : lstAccount )
      {
          List<Contact> lstCont = acc.contacts ;
         
          acc.Number_of_Contacts__c = lstCont.size(); // line 11
      }
      update lstAccount;
  }
}