I want to turn off the two-factor authentication, but there is no option to..

You can have multiple trusted numbers to receive the code on. It does not have to be only your singular phone.

From the support document you came from:

Trusted phone numbers

A trusted phone number is a number that can be used to receive verification codes by text message or automated phone call. You must verify at least one trusted phone number to enroll in two-factor authentication.

You should also consider verifying an additional phone number you can access, such as a home phone, or a number used by a family member or close friend. You can use this number if you temporarily can't access your primary number or your own devices.

That is correct........ you can't.

From the Apple site:

"Can I turn off two-factor authentication after I’ve turned it on?

If you already use two-factor authentication, you can no longer turn it off.

Certain features in the latest versions of iOS and macOS require this extra level of security,

which is designed to protect your information. 



If you recently updated your account, you can unenroll within two weeks of enrollment.

Just open your enrollment confirmation email and click the link to return to your previous security settings.

Keep in mind, this makes your account less secure and means that you can't use features that require higher security."

Which is why you should set a secondary method to receive the 2 factor code. The phone number of a family member, or even a free google voice number, which can accept text messages and be accessed from any device with a browser.

Why do you need to turn it off?

In daily functions it is transparent. It only kicks in when signing in to your account on an unknown device, mail app or browser.