From the US Consulate:
Tourists to Mexico wishing to travel beyond the border zone with their
car must obtain a temporary import permit or risk having their car
confiscated by Mexican customs officials.
To acquire a permit, one must submit evidence of citizenship, title
for the car, a car registration certificate and a driver's license to
a Banjercito branch located at a Mexican Customs office at the port of
entry, and pay a processing fee.
Mexican law also requires the posting of a bond at a Banjercito office
to guarantee the departure of the car from Mexico within a time period
determined at the time of the application. For this purpose, American
Express, Visa or MasterCard credit card holders will be asked to
provide credit card information; others will need to make a cash
deposit of between $200 and $400, depending on the age of the car.
Please do not forget to ask for your receipt from Banjército.
In order to recover this bond or avoid credit card charges, travelers
must return to any Mexican Customs office immediately prior to
departing Mexico. Disregard any advice, official or unofficial, that
vehicle permits can be obtained at checkpoints in the interior of
Mexico. Avoid individuals outside vehicle permit offices offering to
obtain the permits without waiting in line.
If the proper permit cannot be obtained at the Banjercito branch at
the port of entry, do not proceed to the interior where travelers may
be incarcerated, fined and/or have their vehicle seized at
immigration/customs checkpoints.
So it sounds really like as long as you have the ownership papers and registration, it doesn't matter how many cars in your convoy :D
