function custom_limit_lookup() {

    $args = array(
        'fields'     => 'ID',
        'meta_key'   => 'agilecrm_contact_id',
        'meta_value' => '5348394327343104',
    );

    $users = get_users( $args );

    return $users;

}

add_filter( 'wpf_batch_users_sync_init', 'custom_limit_lookup', 20 );