The Pentagon says "more than 7,000" troops are being sent to the Southwest border to support the Customs and Border Protection agents. Officials say that number could reach a maximum of about 8,000 under present plans.
The troop numbers have been changing at a dizzying pace, with Trump drawing a hard line on immigration in the lead-up to the midterm elections.
Trump's new troop estimate caught the Pentagon by surprise. Later, he told ABC News, "We have to have a wall of people."
Troops from 10 states are set to be dispatched to the border in response to a caravan of Central American migrants that is largely composed of families with children and is weeks from reaching the U.S.-Mexico border.
According to the Defense Department, the troops will come from North and South Carolina, Colorado, Illinois, Maryland, Georgia, Texas, Washington, Kentucky and Kansas.
They'll be stationed at an Air Force base and Army installation in Arizona, at several Marine facilities in the San Diego area, and in five locations in Texas, including naval and Army bases.
The document says the Defense Department has identified 7,000 troops who will be participating in the mission at the border. About 2,000 National Guard members are already dispatched at the southern border.