Well I know how to save a list of entities. But just wondering how to save just one

Make an EntityList.

Add the one entity that you want to save to the EntityList.

Save the single entity using this signature: SaveChanges(IEnumerable) Method 

Here is a code example:

private void SaveSample() { 
  PersistenceManager pm = PersistenceManager.DefaultManager; 

  // Retrieve a single Order and its Customer 
  Order order = pm.GetEntity<Order>(new PrimaryKey(typeof(Order), 10275)); 
  Customer cust = order.Customer; 

  // Change the address in both Order and Customer 
  order.ShipAddress = "100 Broadway"; 
  cust.Address = "100 Broadway"; 

  // Now save -- using a save list and SaveOptions. 
  EntityList<Entity> changedEntities = new EntityList<Entity>(); 
  changedEntities.Add(order); 
  changedEntities.Add(cust); 
  SaveOptions options = new SaveOptions(); 
  options.IsTransactional = true; 
  options.ThrowExceptionOnSaveFailure = ThrowExceptionRule.Never; 

  SaveResult sr = pm.SaveChanges(changedEntities, options); 