When Texas Billionaire Nelson Bunker Hunt was testifying at a congressional hearing two years ago, he startled the committee by saying he didn't know how much money he had.

"A fellow asked me that once, and I said 'I don't know,' but I do know people who know how much they are worth generally aren't worth much," said Hunt.

Billionaires may not bother to count their money, but Forbes magazine has done it for them, compiling for the first time a list of the 400 richest people in America.

According to Forbes, the richest man in America is Daniel Keith Ludwig, the 85-year-old New York shipping and real estate magnate, with a personal fortune estimated at more than $2 billion.

Forbes reckons Ludwig has so much money that he remains the richest American even after losing $1 billion trying to start a paper-making empire in the Amazon Jungle of Brazil.

There are more than a dozen other U.S. billionaires, the magazine reports in its Sept. 13 issue. They include Bunker Hunt and four of his brothers and sisters; Perry Richardson Bass and his son Sid; two other Texas oilmen worth $1 billion apiece; Gordon Peter Getty, son of the founder of Getty Oil Co.; David Packard, founder of Hewlett-Packard Co.; Marvin Davis, the Denver oil man who owns 20th Century Fox; Philip Anschutz, another Denver oil and real estate investor; and Forrest Mars Sr. of McLean, former head of Mars Inc., the world's largest candy company.

Mars--the richest resident of the Washington area--is so fanatically reclusive that no picture of him has been published in Washington. That is typical of the megamillionaires on the Forbes 400 list, most of whom refused to cooperate with the magazine's survey.

It took a year to glean the list of the really rich from stockholdings and real estate records, wills, biographies and business experts. The magazine plans to update the list annually, which should sell enough extra issues to enrich publisher Malcolm Forbes, who already qualifies for his own richest list.

Flamboyant publisher Forbes would not reveal the size of his personal fortune to his own publication. The editors included him anyway but, unlike everyone else on the list, they gave no estimate of his wealth.

The poorest person on the list of 400 fabulous fortunes is Armas C. Markkula Jr., 40, of San Francisco, who helped start a company called Apple computer in 1976 and now owns stock worth $91 million. At 27, Apple co-founder Steve Jobs is the youngest person on the list, with $100 million worth of Apple stock.

The census of the really rich stretches from young, self-made microprocessors millionaires to the scions of America's old-money aristocrats, a dozen du Ponts and du Pont relatives, a rook of Rockefellers, a carload of Fords, the Kennedy family, plus assorted Mellons.

At least one of the 400 richest people in America is in a mental hospital--H. L. (Hassie) Hunt III, 65, whose share of his father's estate is worth $600 million, but who has been hospitalized for some 20 years. Another is in exile--fugative financier Robert Vesco, who had $235 million when he fled fraud charges but whose remaining wealth depends on how much it has cost him to avoid the law.

Crime sometimes does pay, the compilation suggests, for the list includes Meyer Lansky, 80, of Miami, listed as "mob money man" with $100 million to his name, and Morris (Mo) Dalitz, 83 of Las Vegas, identified by the Kefauver crime commission in 1951 as a founder of a national crime syndicate.

So does entertainment: Bob "Thanks for the Memories" Hope is listed with $280 million. The late Beatle John Lennon left $150 million to Yoko Ono. Walt Disney's brother Roy is worth $150 million, Forbes figures.

Despite his Playboy image, Hugh Hefner does not qualify for the list. But Penthouse magazine publisher Bob Guccionne--who decided Hefner wasn't giving purient readers enough of what they wanted--makes it with a $200 million fortune.

Other publishers in the Forbes 400 range from the Pulitzers of St. Louis, whose prize is synonymous with newspaper excellence, to Generoso Pope Jr., whose National Enquirer symbolizes a different kind of journalism.

Washington Post Co. Chairman Katharine Graham, with family stockholdings worth more than $100 million, is among a dozen D.C., Maryland and Virginia residents ranked among the 400 richest Americans. Mrs. Graham's brother-in-law William, of Miami, also is listed with his own $100 million fortune earned independently in Florida real estate.

Riggs National Bank Chairman Joe L. Allbritton, who lives in Washington and Houston, is credited with a $250 million fortune, including his share of the Washington's biggest bank holding company and WJLA-TV.

Ranking behind the Mars family in the list of Washington's wealthy are Paul Mellon of Upperville, who inherited $250 million from his legendary financier father 40 years ago and has since doubled his money, and Washington Redskins owner Jack Kent Cooke, who started out selling encyclopedias during the Depression and who now is another $500 million man.

J. Willard Marriott Sr. is estimated to have a $200 million fortune, while the local minor multi-millionaires include former ambassador W. Averell Harriman, $100 million; Athalie (Joan) Irvine Smith of Middleburg, $110 million; Catherine Mellon Conover, Paul Mellon's 40-year-old daughter, whose trust funds total $130 million; and John Kluge, who built a Washington radio station into Metromedia Inc., $140 million.

Not surprisingly, 50 of the 400 richest people in the country are Texans, or at least live in the Lone Star State part of the year. Mere millionaires are so common in Texas that a $10 million fortune has become the standard "unit" by which wealth is measured.

In big-bucks Texas talk, a $100 million fortune is "10 units" and half a billion dollars is just "50 units."

The magazine, which calls itself "a capitalist tool," figures that the free enterprise system has created more than 25 fortunes of more than $500 million.

Runners-up to the $1 billion barrier, in estimated order, are David Rockefeller, of Chase Manhattan Bank, nearly $1 billion; Walter H. Annenberg, founder of TV Guide, $800 million; Stephen Bechtel Sr. and his son Stephen Jr., of the California construction company, almost a billion between them; Harry Helmsley, a New York real estate man, $750 million; Sam Walton, an Arkansas discount-store king, $690 million; William R. Hewlett, co-founder of Hewlett-Packard, $650 million; Edward J. DeBartolo, a Youngstown, Ohio, shopping center builder, $500 million; Samuel J. Newhouse Jr. and his brother Donald, $600 million; Hassie Hunt, $600 million; William Caruth Jr., Dallas real estate, $600 million.

Cyril Wagner Jr. and Jack Brown, co-owners of a Texas oil company worth $1.1 billion; Henry L. Hillman, Pittsburgh banker, $500 million; Paul Mellon; four grandchildren of Houston wildcatter Hugh Roy Cullen, who share $2 billion; A. Alfred Taubman, the Michigan developer who built the Lakeforest and Fair Oaks shopping malls here; Barbara Cox Anthony and Anne Cox Chambers of Atlanta, heirs to the Cox newspaper and broadcasting fortune.

Leonard N. Stern, son of the founder of Hartz Mountain pet food, $500 million; Richard Mellon Scaife, another Mellon heir, who "vigorously denies" Forbes' estimate of a $500 million fortune; Robert O. Anderson, chairman of Atlantic Richfield, $500 million; Curtis Carlson of the Minneapolis trading stamps and hotels businesses, worth $500 million.

Jack Kent Cooke, $500 million; Trammell Crow, Dallas real estate developer, $500 million; Kenneth W. Ford, Oregon lumber baron, $500 million; Samuel J. LeFrak, New York real estate, $500 million; and J.R. Simplot, the Idaho potato grower who holds the patent for making frozen french fries, $500 million. CAPTION: Picture, DANIEL KEITH LUDWIG . . . tops after $1 billion loss