can a passcode be 8 alphanumeric on an IPHONE12

Yes. At one time the maximum was 34 characters but I am not certain about the current maximum. To set an alphanumeric passcode, see Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Yes, choose Custom Alphanumeric Option on New Passcode screen then choose 8 characters that you want.

Set a passcode on iPhone

https://support.apple.com/en-ie/guide/iphone/iph14a867ae/ios