Barely 25 million people watched. Hollywood Reporter:

President Barack Obama’s speech about withdrawing troops from Afghanistan on Wednesday was watched by 25.4 million people — making it the smallest television audience for a major speech since he took office.
It was carried live on nine networks, and scored a 16.4 household rating. As Yahoo! New’s Cutline blog points out, that’s far lower than his December 2009 speech on Afghanistan, which drew 40.8 million viewers, according to Nielsen stats. His speech in August 2010 on the end of the Iraq War attracted 29.2 million viewers, the Gulf Coast oil spill in June 2010 was watched by 32.1 million.
His speech about American involvement in Libya in March was close, with 25.6 million viewers, but, as Yahoo! points out, it was carried on eight networks and aired at 7:30 p.m., before primetime. By comparison, his first appearance on 60 Minutes after the 2008 election attracted 25.1 million viewers.
His most-watched speech was in May when 56.5 million tuned in to hear that Osama Bin Laden had been killed.