Despite President Reagan's claims that there have been too many unauthorized leaks of classified information in his administration, only about "half a dozen" have been reported to the agency in charge of such matters during the past three years.

That estimate was supplied by Steven Garfinkel, the director of the government's Information Security Oversight Office (ISOO), under questioning yesterday at a session of two House subcommittees.

All "knowing and willful" leaks of classified information are supposed to be reported to Garfinkel's office, which is responsible for overseeing the security of all executive branch agencies that originate or handle classified material.

Rep. Don Edwards (D-Calif.), chairman of the House subcommittee on civil rights, expressed surprise at the figure in light of Reagan's directive last month requiring all government employes with access to classified information to submit to polygraph, or lie-detector, examinations if asked during an investigation to track down a leak. That order applies to hundreds of thousands of federal workers, Edwards said.

Garfinkel said he doubted that his office was notified of all the leaks that have taken place since he became ISOO director in May, 1980, during the Carter administration. But he said later that the last one he was told about took place "probably in the middle of 1982."

"Most of them came from military agencies," he said. "Some ended up being reported in the press. Some didn't."

By contrast, Reagan told newspaper publishers in New York this week that leaks of national secrets have "endangered" foreign relations. Similarly, an interdepartmental study that served as the basis for his anti-leaks directive contended that unauthorized disclosure of classified information has become "increasingly common" and was now "a routine daily occurrence in the United States."

"It is not a new phenomenon, but its severity has increased greatly over the past decade," the report said.

The official in charge of the report, Deputy Assistant Attorney General Richard K. Willard, was the first witness at yesterday's hearing. He acknowledged that no special study of leaks was undertaken to show the need for Reagan's directive. Willard indicated that the interagency panel he headed started out on the assumption that it was a longstanding problem that needed to be addressed.

"We have never suggested that it's a problem that has increased greatly in severity in recent years," Willard testified. "It's always been a problem."

House civil service subcommittee Chairman Patricia Schroeder (D-Colo.) said she feared the directive would result in selective enforcement against administration critics while "friendly" leakers would go unpunished and uncensored.

Willard said that was not the goal. He has said even top White House aides will be required to abide by the new directive. In addition to polygraph tests in leak investigations, the order provides for prepublication censorship of the writings of those with access to especially sensitive information.