I’d double or triple that estimate of 20 million illegal aliens. Not all cross the border illegally. An increasing number overstay visas.
Per Bear Stearns study published in 2005, the number then was ~20 million and 3 million more were being added each year. The study was conducted to identify untapped markets for their financial services.
http://web.archive.org/web/20090318213140/http://www.bearstearns.com/bscportal/pdfs/underground.pdf