how to call model method in the model relationship method in laravel - laravel

Question

I have four tables namely: users, games, activities. The users and activities tables have Many to Many relationship. And, games and activities have One to Many relationship respectively. Of course I've that fourth pivot table for the Many to Many relationship above, namely, activity_user.
Inside the Game model, I have this relationship method to call all activities owned by that game, like so:
/**
* Get game activities
*
* #return hasMany
*/
public function activitiesFilter()
{
$user_id = auth('api')->id();
return $this->hasMany(Activity::class)->where('status_id', 2)->multiplayer()->whereDoesntHave('users', function($q) use ($user_id){
$q->where('user_id', $user_id);
});//select only this game approved(status_id===2) activities whose multiplayer count have not been reached and which this user have not joined/created
}
To select activities whose multiplayer count have not been reached, I've a scope function inside the Activity model, like so:
/**
* Scope a query to only include activity whose multiplayer count have not been reached.
*note that the activities table have an int field for multiplayer.
* #return \Illuminate\Database\Eloquent\Builder
*/
public function scopeMultiplayer($query)
{
return $query->where('multiplayer','>', $this->users()->count());
}
However, this doesn't seem to work as $this->users()->count() seems to always return 0.
Your solution will be highly appreciated.