2FAuth/app/Http/Resources/GroupResource.php

<?php

namespace App\Http\Resources;

use Illuminate\Http\Resources\Json\JsonResource;

class GroupResource extends JsonResource
{
    /**
     * Transform the resource into an array.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'id'                    => $this->id,
            'name'                  => $this->name,
            'twofaccounts_count'    => $this->twofaccounts()->count(),
        ];
    }
}