(Reuters) - U.S. auto safety regulators said on Tuesday their estimate for the millions of vehicles affected by the Takata Corp (>> Takata Corporation) air bag recall will likely be revised because cars with two front air bags were double-counted.

The National Highway Traffic Safety Administration has come under fire for failing early on to catch the defective Takata air bag inflators, as well as faulty ignition switches in General Motors Co (>> General Motors Company) vehicles. Both high-profile recalls pushed the agency into the spotlight.

NHTSA Administrator Mark Rosekind told the Senate Commerce Committee on Tuesday that regulators are waiting for automakers to provide information including the number of Takata inflators that must be replaced more than once. Rosekind cited his agency's underfunding, low staffing and lack of authority as some lawmakers criticized NHTSA.

“You’ve got too many complaints and not enough people,” he said. “It’s just overwhelming.”

Senator John Thune, Republican of South Dakota, said NHTSA's problems will not be solved purely with additional resources. Rosekind said he would provide a list of 44 changes the agency is implementing to improve effectiveness, and Congress needs to give NHTSA more power to compel recalls.

The Obama administration said last month it was doubling the number of vehicles involved to nearly 34 million, making the Takata recall the largest in U.S. history. But a Reuters analysis found the number could prove to be less than half that.

Rosekind, who testified at the same hearing as representatives from Takata, automakers and the Department of Transportation, said on Tuesday there are about 34 million defective inflators in 32 million vehicles on U.S. roads that need to be replaced.

"It is important to note that this number is an estimate and will be refined," he said. "We know that there are almost certainly vehicles that are counted twice."

Takata air bag inflators can deploy with too much force and spray metal shrapnel into passenger compartments. They have been linked to at least eight deaths and more than 100 injuries.

A day before the hearing, committee Democrats released a report saying the Japanese company may have put profits over safety by halting global safety audits for financial reasons.

"This is deadly serious business," said Senator Bill Nelson, a Florida Democrat, as he held up a piece of shrapnel from a Takata air bag that injured a Florida woman whose bloody, gauze-wrapped face wrapped was displayed in a photograph at the hearing. "For years, it's obvious that Takata did not put safety first."

Senator Richard Blumenthal of Connecticut compared the eight deaths currently linked to Takata air bags to the 13 deaths initially cited by GM over its defective ignition switches. Since then, the number has risen to 117 deaths, according to a lawyer working for GM. Blumenthal said the Takata number could also rise.

Also testifying are Fiat Chrysler  (>> Fiat Chrysler Automobiles NV) and Honda Motor Co (>> Honda Motor Co Ltd), which use the Takata airbags in their vehicles.

Fiat Chrysler said in written testimony that it was replacing all driver-side air bag inflators affected by the Takata recall with products from TRW Automotive as of June 8.

(Reporting by David Morgan in Washington and Ben Klayman in Detroit; Editing by Lisa Shumaker and Matthew Lewis)

By David Morgan and Ben Klayman