Recovery key

Once you generate a recovery key you alone are responsible for your account security. From→ How to generate a recovery key - Apple Support:

Using a recovery key is more secure, but it means that you’re responsible for maintaining access to your trusted devices and your recovery key. If you lose both of these items, you could be locked out of your account permanently. With that in mind, it's important to keep your recovery key in a safe place. You might want to give a copy of your recovery key to a family member, or keep copies in more than one place. That way you always have your recovery key when you need it.

You remember it or find where you wrote it down.