Adjust query

You can adjust the query for migrations by overriding the query function.
Below are some examples to limit the amount of nodes you migrate.

protected function query() {
    $query = parent::query();
    //$query->condition('n.nid',2161); // only migrate the node with nid 2161
    //$query->condition('n.nid',array(1849,2161)); // only migrate the nodes with nid 1849 or 2161
    $query->range(0, 50); // only migrate the first 50 results
    return $query;
}

For more information about the syntax check the database api documentation.
http://drupal.org/developing/api/database