Suppose they had a disco and nobody came? That's how last Sunday night started at Studio 54 in New York, the world's largest, flashiest hangout for the headline grabbers. Before midnight there were nearly as many groupies outside the former television studio as there were clients inside, where the mostly male crowd had the basketball court-size dance floor to themselves.

"Even Disneyland has slow days, y'know," teased Billy Patton, a manager at the disco. Not a bad analogy. Barechested waiters in jogging shorts and sneakers danced with their brooms alongside the couple in New York Yankee suits, the women in leotards or opaque pantyhose with oversized blouses and spike heels, the men in jeans and T-shirts. At the bar - plenty of cowboy boots, some with spurs, resting on the bar rail.

Soon after midnight, the start of the regulars. First, designer Charles Supon, who just moved to Central Park West so he could be within walking distance of 54, in his "Studio uniform" - a Lacoste shirt, tight jeans, and cowboy boots. Another familiar face on the dance floor - Washington store withdow designer Brian Lause, who had taken the last shuttle up from D.C. that night. "I was in the Lost and Found and really bored, so I grabbed some friends, used a little plastic and here we are."

By half past midnight, Disco Sally, 77-year-old Sally Lippman, who is the current media star, was surveying the scene from one of the silver-covered, sausage-shaped banquettes, surrounded by five or six young male admirers. "I gotta come every night. It's like a drug," she said later as she leaned against he bar, almost smothered by John Touzos, 25, who Sally called a "working playboy" but who actually manages a boutique and works a couple of days a week as a maitre d', and whom she says she may marry (at Studio 54) this fall. "She deserves it," said Touzos about the merger with his intended. "She's just a young woman wrapped up in an old body."

Sally Lippman is a lawyer. "I haven't practiced for 40 years but have given out advice free, gratis, for nothing," she said. Her husband, who died three years ago, had studied law and had worked for the Federation for Jewish Philanthropies and in public relations. Her husband never really liked to dance. They both liked "good music."

Last September, she first came to Studio 54 "with a yound 25-year-old boy" who told her, "You really must see it." She recalled, "I stuffed my ears with cotton, and came."

It easy trading mah-jongg pieces for evenings at Studio 54. She's always had gay friends. Her husband knew that. "There is something about gay people . . . they are all a little irresponsible," she said, looking up from her tequila and orange juice. "I like that, I find straight people very dull."

Sally, who had just the week before been in Boston raising money for educational TV, and has been featured in New York magazine, said she jokingly told Liza Minelli, another 54 regular, that she and Touzos might be married and that is how she came to thinking about it seriously.She's met Touzos' sister from Houston to tehh her about the wedding and Sally reported, "she said, of course 'I don't mind.'" Added Sally, "I can't imagine why she feels that way." (Apparently Sally's family doesn't feel the same way. On the Today show recently she said that her family heartily disapproved but "I'm having the time of my life." Later she added, "What's the matter with dying on the dance floor."

Now Sally is back on the dance floor, dancing with her legs wrapped around one of her entourage, her arms hugging his neck. Her red framed glasses tilt to one side as he twirls her, but she doesn't seem to mind.

Back to the bar. "I'll celebrate my 78th birthday here in October, then my wedding, maybe." Then Sally leaned over to whisper to her interviewer. "He doen't believe in sex before marriage. Isn't that nice."

One way to count on a crowd at a disco is to let someone else throw a party and bring his friends. Recently at Xenon, the newest entrant in the disco derby in New York, there was a party and therefor a full house, honoring ex-champ Muhammad Ali. Last week on Monday, mannequin mogul John Casablancas, threw himself a party to celebrate his first year in business and a new liaison with a television talent company. Casablancas' agency, Elite, will have billed $4 million plus the first year, 25 percent of it in profit to the owner, he said.

There were 2,500 invites sent out, each for two persons. At least that many came and were admitted, the crowd body to body in the barely air-conditioned room. Good Humor giveaways, courtesy of Casablancas, from a truck parked in front of the former theater, did a meager business until they decided to feed the crowd lined up to get into the party.

Casablancas likes his models to be 16-to 22-year-olds, 5 feet 7 to 5 feet 11, with regular feature, "and a kind of baby roundness." He said he stops people on the street occasionally to tap them for modeling. The agency handles 150 models, 35 of them male, who are most successful these days between ages 30 and 35 with a rugged macho look, "but they can be gay as long as they don't look like it. That's their problem." Several top Elite models including Iman, Beverly Johnson and Christie Brinkley never showed, but Yasmine (the Bloomies bag girl) was there in corset top and black tiered skirt designed by Michael K. Borden. So were Lari Taylor in brocade kimono (she's a recent Seventeen magazine cover girl) and Aria Riccardo, 16, touted as the upcoming No. 1 black model, wearing a schoolgirl blouse.

Casablancas expects a decision soon on the $7.5 million lawsuit by model agency magnate Eileen Ford, for pirating top models and other employes of the Ford agency - naturally in his favor. "They I can sue everyone for the fortunes I've spent in legal fees."

Wanna know why it is hard to take the fashion industry seriously? Designer Halston, a Studio 54 fixture, gave a return party for a select group of 54 employes and the inner circle of regulars last Monday night . . . They were to dress as some 54 patron - of the opposite sex. Studio 54 owner Steve Rubell led the pack wearing a Liza Minelli Halston-designed costume for "The Act," Jewelry designer Kenneth Jay Lane borrowed Diana Vreeland's lip gloss to wear with his black satin pajamas ("I was going as DD Ryan." Vreeland insisted he looked more like a maharajah. Lane reported.) Disco Sally was gussied up in a red Ari Onassis suit and her "fiance" Touzos went as Jackie Onassis in a white Halston and spike heels. Fashion model Barbara Allen wore a padded jock strap, according to one report. Host Halston, as a socialite desiger, greeted guests in one of his own one-shoulder numbers.

Said a 54 employe about the party. "I guess he asked us to dress up so we'd feel more comfortable in his lavish home.