The revised number came from the National Highway Traffic Safety Administration, which said about four million of the vehicles have defective airbag inflators on both the driver and passenger sides.
Takata's defective airbags have been blamed for eight deaths and more than 100 injuries around the world.
The defect -- thought to be associated with a chemical propellant that helps inflate the airbags -- can cause them to deploy with explosive force, sending metal shrapnel hurtling toward drivers and passengers.
The airbag recall -- the biggest product recall in US history -- poses a challenge in execution. NHTSA said it has consulted with all the vehicle manufacturers and numerous air bag suppliers "to gather information on inflator supplies, risk factors, and the biggest obstacles to replacing defective inflators."
It also said it had completed its own initial testing of Takata inflators, undertaken to compare with test results from Takata and others.
"Preliminary results are broadly consistent with data from Takata, including Takata's findings on the risk associated with vehicles from high-humidity geographic areas," the agency said.