NEW YORK (AP) _ Many CIA officials ''papered over their consciences'' about estimating Vietnamese communist strength after the powerful Tet offensive, a CIA official testified in retired Gen. William Westmoreland's $120 million libel suit against CBS.

Richard Kovar, who until two weeks ago wrote President Reagan's daily intelligence briefing, on Wednesday became the latest in a string of CIA staffers from the Vietnam era to testify in the network's defense.

Unlike most earlier witnesses, Kovar remains on active duty with the agency, though he did not state his current assignment.

Westmoreland, who commanded U.S. troops in Vietnam from 1964 to 1968, claims his reputation was damaged by false charges in a 1982 documentary, ''The Uncounted Enemy: A Vietnam Deception.''

The CBS broadcast charged that Westmoreland, in an effort to show his troops were winning the war, in 1967 suppressed evidence showing communist forces were much larger than had been believed.

The enemy strength estimates were the subject of a lengthy bureaucratic dispute in the summer of 1967, with the CIA arguing for a total above 500,000 while Westmoreland's staff insisted on a figure below 300,000.

The dispute was resolved in September 1967 when George Carver, a special assistant to CIA Director Richard Helms, accepted the military's figures with minor modifications. A ''special national intelligence estimate'' submitted to President Johnson and other leaders in November put communist strength at 242,000 troops.

That report ''did not describe or estimate accurately or completely the capabilities of the Vietnamese communists we were fighting,'' Kovar said Wednesday.

Two months later, the Tet offensive began. CBS maintains American troops and their leaders were caught off guard when the communists attacked all of South Vietnam's major cities simultaneously.

''Our failure to do a better job in the estimate had a direct consequence in the events in Vietnam,'' Kovar testified. ''A lot of people in the CIA had papered over their consciences on this subject. I myself didn't feel too good about this.''

Another ex-CIA analyst, Joseph Stumpf, testified today that the enemy strength estimate ''wasn't really an accurate portrayal, but a set of contrived numbers.''

Stumpf said he was assigned in 1967 to determine the number of ''administrati ve'' soldiers in the communist forces, and that his estimate of 75,000 to 100,000 was ''sound and conservative.'' Westmoreland's command, and the special national intelligence estimate, put the figure between 35,000 and 45,000.

On cross-examination Stumpf conceded that he did not attend many of the meetings where the evidence was reviewed and that the CIA, in its own study completed in May 1968, trimmed his estimate to 60,000 to 80,000.

Kovar strongly defended Samuel A. Adams, a former CIA analyst who left the agency in 1973 and went public two years later with charges that data on enemy strength had been improperly withheld. Kovar said he encouraged Adams in the late 1970s to write a book on the subject.

Adams, who has written the book but not published it, became a consultant to CBS and is a co-defendant in the suit.

CBS lawyer Robert Baron asked Kovar, a former CIA deputy director for intelligence, to respond to suggestions by Westmoreland's lawyers that Adams is a ''mental case.'' Adams spent nearly a decade after leaving the agency trying to put his story before the public.

''I have never observed anything whatsoever to justify that characterization, '' Kovar replied. ''What Sam did wrong was that he didn't salute and shut up.

''He didn't close ranks. Not only didn't he shut up, he pushed his arguments and he pushed his outrage at the agency's acquiescence ... beyond where a subordinate was supposed to go. And that frightened a lot of people at the agency, and it made them mad.''