Every day the the border patrol station at Douglas arrests about 600 migrants attempting to enter the US illegally. The biggest and busiest in the country, it covers six and a half miles centred on the border crossing between Douglas and the Mexican town of Agua Prieta.
http://www.guardian.co.uk/usa/story/0,12271,1243582,00.html

In the Tucson sector, which covers eight stations including Douglas, 366,043 immigrants have been detained since the beginning of October, compared with 233,035 in the preceding 12 months.

Some say 80% of those trying to enter the US illegally are successful.

Unless they have criminal records in the US, or can be proved to be citizens of countries other than Mexico, and thus liable to be deported to their country of origin, they will be put on a border patrol bus, driven the four miles to the border crossing and released, free to try the crossing again, perhaps the same night.