source: database/seeds/PostsTagsTableSeeder.php

<?php

use Illuminate\Database\Seeder;
use \Illuminate\Support\Arr;
use App\Models\Post;

class PostsTagsTableSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        $posts = Post::all();

        $posts->each(function($item, $key) use($posts) {
            $item->tag()->attach(Arr::random($posts->pluck("id")->toArray(), rand(2, 5)));
        });
    }
}