A new report finds the Homeland Security Department "grossly underestimates" the number of illegal aliens living in the U.S. Homeland Security's Office of Immigration Studies released a report August 31 that estimates the number of illegal aliens residing in the U.S. is between 8 and 12 million. But the group Californians for Population Stabilization, or CAPS, has unveiled a report estimating the illegal population is actually between 20 and 38 million.