Is there a hook that is called only once after plugin bulk update process is completed?

I think the action you’re looking for is indeed upgrader_process_complete

The Codex page doesn’t say specifically, so I’m not totally certain if it runs for each plugin or after all of them, but it works for me 🙂

function bgmc_plugins_update_completed( $upgrader_object, $options ) {

    // If an update has taken place and the updated type is plugins and the plugins element exists
    if ( $options['action'] == 'update' && $options['type'] == 'plugin' && isset( $options['plugins'] ) ) {
        // code here
    }
}
add_action( 'upgrader_process_complete', 'bgmc_plugins_update_completed', 10, 2 );