I am not sure if each state within Mexico has it's own law in regards to the number of plates your car needs or if it is a Federal law. I do know that in the state of Yucatan you are only required to have one plate. However, from my conversations with other expats, if you only have one plate you are more likely to be waved over for inspection at roaming state police check points simply because they can't size you up. Since they can't tell if your car has plates until you pass them they are more likely to have you pull over, look at the back of your car and then decide if they are ready to let you pass.
This is what I have heard form others. So.......A mutual Texas friend of mine who lives here in the Yucatan showed me how to make a plate at Office Depot or Office Max. Cost.....In the states it will run about $10 USD. My friend made his after he lived down here for a while.
Step One: Take your original plate off your car and take it into Office Max or Office Depot. Have them take a photocopy of your license plate on color copying, not black and white.
Step Two: Have them laminate the photo with a plastic laminator. Then have the lady there take a razor blade or large paper cutter and shave the edges off of it until it is the exact size as your original plate.
Step Three: Take the most durable plastic/foam poster board you can find in the store. Look around until you see something that is non-paper and looks like it has some weather durability.
Step Four: Have them take a spray adhesive and spray it on the back of your laminated photo of your plate. Place the photo onto the plastic poster board. Then have them cut the poster board around the edges of your laminated picture until everything is the exact same dimensions as your original plate.
Step Six: Buy four mounting bolts about 2 & 1/2 inches to 3 inches long from your local Home Depot along with 4 washers and 4 nuts for the bolts. Also you will need a drill and a drill bit just slightly larger than your mounting bolts.
Step Seven: Line the plate with the plate holder up where you want it on your front bumper. Drill some guide holes through your license plate into your front bumper. Then remove your plate and finish drilling your holes. When finished mount your new plate and step back from the car. You be amazed how real it looks.
I would install it right before you drive over the border into Mexico so you are not driving reasonable distances in the United States with a pseudo-plate. Wal Mart parking lot off of the main Highway heading south in Brownsvile, TX is a good location. I am sure there are laws against doing this in the United States so if you have the chance and the money just skip everything and get a real plate. We did this for my friends truck because we were in a pinch and I didn't notice that he needed to do this until we were standing in Houston, TX.
However, if you can't get a real plate and you are reduced to doing it this way then, when you get done you are not going to be able to tell the difference between your newly made plate and an original from your state's Department of Public Safety, especially after it is covered with bug splatter.