Running a loop whilst seeding db

Sure it is. I do it with this code in my seeder.

 class AuthorsTableSeeder extends Seeder
{
    protected $authors = [
        [
            'first_name' => 'Robert',
            'last_name' => 'Jordan',
            'slug' => 'jordan-robert'
        ],
        [
            'first_name' => 'Matt',
            'last_name' => 'Stauffer',
            'slug' => 'stauffer-matt'
        ],
        [
            'first_name' => 'Brandon',
            'last_name' => 'Sanderson',
            'slug' => 'samderson-brandon'
        ]
    ];

    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        foreach($this->authors as $author) { Author::create($author); }
    }
}

No, I mean like this:

for($i = 0; $i > 100; $i ++)
{
DB::table('users')->insert([
            'name' => str_random(10),
            'email' => str_random(10).'@gmail.com',
            'password' => bcrypt('user'),
            'is_admin' => 0,
            'created_at' => date("Y-m-d H:i:s"),
            'updated_at' => date("Y-m-d H:i:s"),
        ]);
}

Then you should use factories

$factory->define(App\Author::class, function (Faker $faker) {
    $firstName = $faker->firstName();
    $lastName = $faker->lastName();
    return [
        'first_name' => $firstName,
        'last_name' => $lastName,
        'slug' => Str::slug($lastName . ', ' . $firstName)
    ];
});

factory(Author::class)->create();

To create more than one just pass the number as a second argument.

factory(Author::class, 50)->create();

https://laravel.com/docs/6.x/database-testing#writing-factories