I think this should do it for you:
According to INEGI (National Institute of Statistics, Geography and
Information in Mexico), the number of workers in foreign-owned
(chiefly US-owned) plants has almost doubled in the past decade, from
648,000 workers in 1995, to 1,167,000 workers in 2005. The number of
US-owned plants in Mexico has increased steadily, from 1,700 such
plants in 1990, to more about 3,800 plants in 2001, according to the
US Environmental Protection Agency.
[...The rate of industrial development increased further after the
North American Free Trade Agreement (NAFTA)*, with about 1,700 plants
operating in Mexico in 1990. By 2001, that figure had more than
doubled to nearly 3,800 maquiladora plants]
INEGI (National Institute of Statistics, Geography and Information)
Let me know if there's anything else you need on this.
search strategy -- Google searches on:
" US owned" plants OR factories mexico
maquilas OR maquiladoras
"1500..5000 maquilas OR maquiladoras"