List all the users with the count of drivers they registered

i have tow models Userand Driver the user can register any number of drivers drivers

in drivers table i have agent_id, i want to list all the users with the count of the drivers they registered

in my Usercontroller i want to get the count of the drivers registered by the agents

public function index(Request $request) 
{
    $users = User::all();

    $registration_centers = RegistrationCenter::all();

    $drivers_registered = Driver::where('agent_id' , $agent_id)->count();  //something like this but i don't have agent_id to query with

    return view('agents', compact('users', 'registration_centers', drivers_registered));
}

thank you