I just bought iPhone 12 with 64gb storage. Do I need to continue to pay for 50gb iCloud storage also?

iCloud storage and iPhone storage are different things. For different purposes all together. Its entirely up to you wether you want to continue paying for it or not.

Anything you have in iCloud that exceeds 5gb will be deleted from iCloud if you cancel the storage.

Consider wether or not you want to continue to sync things to iCloud and store your device Backups, before cancelling it.