The consensus seems to have fallen on vehicle license plates / registration tags / whatever they're called wherever you might be, and it was also my first guess, even though I'm not from the US. That said, I do watch a few things on YouTube that might have primed me to think of those first and not any other kind of plate.
I don't understand what OP is asking either, even with the upvoted response to your comment. What do Mississippi license plates look like?
I live in a different state, and we have the option to have either our state's motto on our plate, or the county that we live in. I chose the county that I live in. When I drive around, I see many others displaying the county that they live in as well. Is Mississippi's system similar to this? I don't understand why it would be a hassle on the part of the state to print different license plates for different people. They already do that anyway.... I don't get how that would be using excessive resources.
When I have to renew my vehicle registration, it is either done online, within kiosks at certain grocery stores, or at a government building within the county I live. It doesn't seem like a confusing or weird system at all to me. But again, I don't know how it works in Mississippi.
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.