Number of unauthorized immigrant population. The lessen available jobs in the US. have also lessen the number of illegal migrant. DHS released a report that the illegal migrant, population living in the U.S. lessen to 10.8 million. In January 2009 and grew by 27 percent between 2000 and 2009. Unauthorized Immigrant Population. Border Patrol apprehensions of migrants at the U.S.-Mexico border are on the rise again. Although the majority of people attempting to enter the United States illegally are stopped, this trend could foreshadow an increase in the U.S. number of unauthorized immigrant population after years of relative stability. Yet the activity at the southwestern U.S. border is only one part of the overall story of unauthorized immigration, as a growing share of this population came from regions other than Mexico or Central America and entered the U.S. legally but overstayed their visas. The unauthorized immigrant population is always changing and churning.