Data Mapper instance to save and retrive multiple objects of the same type

A Data Mapper is meant to:

Move data between objects and a database while keeping them independent of each other and the mapper itself. - Martin Fowler

Most Data Mappers I work with only deal with and return a single object instance.

Is it okay use ONLY ONE Data Mapper to retrieve, manipulate and save multiple objects of the same type?

Example:

public class UserController {

    public function index() {

        UserMapper $um = new UserMapper();

        User $u1  = $um->getById(34);
        User $u2  = $um->getById(23);
        User $u2  = $um->getById(02);

        //Do stuff to user objects...

        $um->save($u1);
        $um->save($u2);
        $um->save($u3);

    }

}