Recovering iCloud account with forgotten 2-factor authentication number

Please enroll in account recovery:

How to use account recovery when you can’t reset your Apple Account password - Apple Support

Prior to embarking on Account Recovery, suggest that you confirm that you indeed CAN’T receive a 2FA verification code via a pre-configured Trusted Number which you might have simply forgotten you had configured.

The “sending” process provides “drop downs” of the number(s) which you have configured.

Select “Didn’t Get a Code?” or “Can’t get to your devices?” on the verification code screen.

Sources:

About trusted phone numbers and trusted devices for Apple Account - Apple Support

and

Get a verification code and sign in with two-factor authentication - Apple Support