If my iphone is lost or stolen and I log into find my phone, the two factor authentication code is sent to my phone. How then do I log into Find my phone?

It's a bad mistake to have 2FA on the iPhone you lost.

Normally you have at least 2 trusted phone number (a normal landline can also be used)

If you still can't sign in

If you can’t sign in, reset your password, or receive verification codes, you can request account recovery to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity.

If you are using a browser make sure to include the “find” in the URL.

If using FindMy on a borrowed Apple device, make sure to click on “Use a Different AppleID” (it’s in blue type at the bottom of the screen)

Try logging into iCloud.com/Find.

See if anything here helps. 

Verification code - how to get one and sign in with two-factor authentication.