Do Consulado dos EUA:
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.
Então, parece que, desde que você tenha os documentos de propriedade e o registro, não importa quantos carros no seu comboio: D