Instead of “number plate”, we call it either a “license plate” or just a “plate”. Some states require both front and rear plates, while others only require a single rear plate.

A “tag” is a small registration sticker that is updated/changed every so many years for proof that your vehicle registration is current. The “tag” is often placed on your license plate itself, but some states put the tag on the windshield instead. Some other states don’t actually have any tag stickers that need to be placed anywhere on your vehicle even though your vehicle still needs to be registered.

I think the confusion is arising because a user here is referring to their license plate as a “tag”. In my experience, this is not common word usage. I’ve never heard of anyone refer to a vehicle tag as anything other than the registration sticker. But just because it’s not common in the places I’ve been doesn’t mean that people don’t use the word that way.