"Measuring migration flows between Mexico and the U.S. is challenging because there are no official counts of how many Mexican immigrants enter and leave the U.S. each year. This report uses the best available government data from both countries to estimate the size of these flows. The Mexican data sources â a national household survey, and two national censuses â asked comparable questions about household membersâ migration to and from Mexico over the five years previous to each survey or census date. In addition, estimates of Mexican migration to the U.S. come from U.S. Census Bureau data, adjusted for undercount, on the number of Mexican immigrants who live in the U.S. (See text box below for more details.)"
The way I understand it 1 out of 5 Mexicans live in the USA.