What's the penalty for having an obscured number plate?

Don't cover, mask, or alter your license plate in any way - most jurisdictions will hit you with a fine ranging anywhere from a couple hundred to over a thousand dollars depending on where you live, and some places can even suspend your registration or vehicle inspection. The specific penalty varies by state or country, so your best bet is checking your local traffic laws or the DMV website to see exactly what applies to you. Intentionally obscuring a plate is taken pretty seriously because it interferes with traffic cameras and law enforcement, so it's not worth the hassle.