Illinois issues 2, but I've seen many cars with only the back one. Most often it's a car that's undergone lots of modifications. I've also seen people with their plates stuck to the inside of their windows, both front and back or just the front one, with the rear in the normal place.
Until very recently--perhaps the past 4 or 5 years--Nebraska was pretty laid-back and lenient about license-plates. Now it's pretty much a hard-and-fast rule that both plates need to be where they're supposed to be.
If either plate--or for that matter, both plates--are inside the windshield or the rear window, one needs to have a good reason (and sometimes there are good reasons) for doing that. If there's a good reason, law enforcement lets it go, no big deal. But if there's not a good reason, some sort of minor ticket.
The bigger deal involves those 8.5x11 paper IN TRANSIT displays.
After purchasing an automobile in Nebraska, one has 30 days to license and plate it.
There always used to be dealer-issued IN TRANSIT displays, and hand-made ones, in the cases of an automobile being a private purchase, and not through a dealer.
Well, there were problems with the home-made signs. Some people, after 30 days had lapsed, just put up a new hand-made IN TRANSIT display, with a new date of expiry. Ultimately, they would get caught, but most of the time one could get away with it for several months before that happened.
So now the rule is only dealer-issued IN TRANSIT displays are allowed.
If one purchases an automobile from a non-dealer, until he gets the thing licensed and plated, he is to drive around with nothing at all (which I myself have had to do), resigned to being stopped at some time. In which case one yanks out the dated bill-of-sale, and it's all copacetic.