If you get pulled over without a license plate then what will happen is you will usually get a fine or ticket for driving without a license plate.
In some cases you may simply get a verbal warning and be sent on your way with the vehicle.
As long as you have auto insurance on the vehicle and you have a valid drivers license and are respectful to the cop that pulled you over they can most often use their discretion.
I've been pulled over without a license plate before and I was told to just get it registered as soon as possible.
I had insurance and a valid license and just had gotten the vehicle and had the Bill of Sale with me.
I just got a verbal warning and went on my way.
Once I also got stopped and pulled over with an expired license plate and was sent on my way with a verbal warning and I went to the DMV that day and got my registration renewed.
It can vary from state to state and the police and how you act.
If you have no insurance on the car and no license plate then your vehicle will likely be towed.
In some states such as Arkansas you have 30 days to register the vehicle once you buy it and as long as you have insurance and your bill of sale you can drive the vehicle 30 days without a license plate.