DEFINITION: Estimated number of Illegal Immigrants. Latest available data - 2000 Census. Eight other States --Maine, Montana, New Hampshire, North Dakota, South Dakota, Vermont, West Virginia, and Wyoming --each had fewer than 2,500 estimated unauthorized residents in 1990 and 2000. The US Citizenship ad Immigration Services also highlights that the illegal immigrant population in America grows by approximatley a half a million each year. Taken into account, the current illegal immigrant population is between 9 and 11 million people.