Factory.php

<?php

namespace LaraShop\Factories\Entities;

use Illuminate\Database\Eloquent\Model;
use LaraShop\Marks\Entities\Mark;

class Factory extends Model
{
    protected $table = 'factories';
    protected $fillable = [];
    protected $appends = [];

    /**
     * @return \Illuminate\Database\Eloquent\Relations\BelongsToMany
     */
    public function marks()
    {
        return $this->belongsToMany(Mark::class, 'factories_marks', 'factory_id', 'mark_id')->withTimestamps();
    }
}

Mark.php

<?php

namespace LaraShop\Marks\Entities;

use Illuminate\Database\Eloquent\Model;
use LaraShop\Factories\Entities\Factory;

class Mark extends Model
{
    protected $table = 'marks';
    protected $fillable = [];
    protected $appends = [
        'business_total'
    ];

    /**
     * Aqui que eu gostaria de usar o "Eager Loading"
     * @return int
     */
    public function getBusinessTotalAttribute()
    {
        $id = $this->attributes['id'];
        
        // Como eu gostaria
        return Factory::with(['marks' => function($query) use($id) {
            $query->where('id', '=', $id);
        ])->count();
            
        
        // Gambiarra
        foreach (Factory::all() as $factory) {
            foreach($factory->marks as $mark) {
                if($mark->id == $id) {
                    $marks[] = $mark;
                }
            }
        }
        return isset($marks) ? count($marks) : 0;
    }
}