Can someone tell me what the screw # and size is for the back license plate holes?

One of the screws is rusted and will only screw in half way before the screw starts to strip. The other is rusty but has no problem, figure time to replace them both, I just don't know the # and size of the screw to buy form a hardware store.

Much appreciated for the help.

 

M6 x 15mm should be fine. 1.0mm Thread pitch

 

You can go with what LBTC said. I myself would just take the screw with me into the store to match it, don't forget to get stainless steel ones if they carry them.