I added the Apex Class and Apex trigger below to update the field Number_of_Contacts at the Account level when a Contact is added or removed to a certain Account:

Class:

public without sharing class  ContactTriggerHandler {
    
   private Set<Id> getAccountIds(List<Contact> contacts) {
       
       Set<Id> accountIds = new Set<Id>();
       
        for(Contact c : contacts) {
            if(c.AccountId != null) {
                accountIds.add(c.AccountId);
            }
        }
        
        return accountIds;
   }
    
   private Map<Id, Account> getAccountMap(Set<Id> accountIds) {
       
       return new Map<Id, Account>([SELECT Id, Number_of_Contacts__c FROM Account WHERE Id in :accountIds]);
   }
   public void process(List<Contact> contacts, System.TriggerOperation operation) {
       
       Set<Id> accountIds = getAccountIds(contacts);
        
        if(accountIds.size() > 0) {
            Map<Id, Account> accountMap = getAccountMap(accountIds);
            
            for(Contact c : contacts) {
                if(accountMap.containsKey(c.AccountId)) {
                    switch on operation{
                        when AFTER_INSERT {
                            accountMap.get(c.AccountId).Number_of_Contacts__c += 1;
                        }
                        when AFTER_DELETE {
                            accountMap.get(c.AccountId).Number_of_Contacts__c -= 1;
                        }
                        when AFTER_UNDELETE {
                            accountMap.get(c.AccountId).Number_of_Contacts__c += 1;
                        }
                        
                        when AFTER_U {
                            accountMap.get(c.AccountId).Number_of_Contacts__c += 1;
                        }
                        
                    }
                }
            }
            
            update accountMap.values();
        }
       
   }


}

Trigger:

trigger ContactTrigger on Contact (after insert, after delete, after undelete) {
    
    ContactTriggerHandler handler = new ContactTriggerHandler();
    switch on Trigger.operationType {
        when AFTER_INSERT {
            handler.process(Trigger.new, Trigger.operationType);
        }
        when AFTER_DELETE {
            handler.process(Trigger.old, Trigger.operationType);
        }
        when AFTER_UNDELETE {
            handler.process(Trigger.new, Trigger.operationType);
        }
    }
    
}

However, how can I include a line of code that updates the Number_of_Contacs__c field when a Contact moves to a different Account?

Thank you,