IN NEW YORK now, or headed that way, are 11 "plays" with a common attribute: mini-casts. In the entire lot are 18 players, not enough to cast "Hamlet."

Okay, so you want to cast "Hamlet." You must also add the casts from three current musicals, with their 10 performers and, as royalty has been doing, cut down on the court, and you have enough players for "Hamlet." Only don't try to do "Antony and Cleopatra," which has 34 characters; "Julius Caesar," which lists 35; or "Troilus and Cressida," which requires, besides 25 principals, Greek and Trojan soldiers.

No, I grant you, it is not a must that a good play have a passel of players. There have been good two-character palys and one-character portraits. But the whole matter of mini-dramas is getting a bit out of hand.

The term "play" is taking a licking. Dore Schary, who did a play on the Franklin D. Roosevelt family in "Sunrise at Campobello," now has "FDR," with Franklin Roosevelt again as his central figure. Billed as "a new play" to open at the National Nov. 8 for a four-week run, it has only one actor, Robert Vaughn. Maybe it will be magnificent. But is it a "play"?

In his admired portrait of Oscar Wilde, "Diversions and Delights," Vincent Price is headed for New York in a vehicle that will serve him and the wide Wilde public well. John Gay has managed to form drama through two contrasting acts, and Price has a role worthy of his veteran, often undervalued skills.

New York's only hit of the season so far seems to be Hume Cronyn and Jessica Tandy in "The Gin Game," a first play by a Baltimore-born Texan, D.L. Coburn. In four scenes Coburn details the lives of two residents of a senior citizens' "rest home," and under Mike nichols' staging the world could well serve Cronyn and Tandy for several years.

Also current in New York is "Miss Margarida's Way," which employs two per formers, although Colin Garrey only mines his role. Estelle Parsons, as a teacher, addresses not merely the theater as a classroom but also as the world in Robert Athayde's tour-de-force.

"A Life in the Theater," at Off-Broadway's Theatre de Lys, is by David Mamet, author of "The Mamet Plays," which had a brief run here last spring, and "American Buffalo," which had three characters. As in "Duck Variations" from "The Mamet Plays," the new play has two characters, both actors, and is a study of their profession's blind cruelties. Ellis Rabb has most of the action, with Peter Evans as his confidant.

James Whitmore reaches New York Tueaday as Theodore Roosevelt in Jerome Alden's "Bully!" There have been some revisions since its National visit last February, and the Panama Canal has become an even hotter topic.

Mary Martin and Athony Quayle approach Chicago with Aleksei Arbutzov's "Do you turn Somersaults?" and heads from New York in January. Thousands enjoyed the play during the summer at the Eisenhower. Martin and Quayle were the only players, but 19 pairs of hands were working in the winds.

Prepping for the National is James Early Jones as "Paul Robeson," by Phillip Hayes Dean. Another solo Roosevelt drama "Pencilled in "Form New York is arlene Stadd's "Eleanor," which starred Eileen Heckart last Year at Ford's.

With those shows, the long-running, "Same Time, Next Year" with its two characters, and "Vanities," with three, we have 11 offerings with 18 performers.

The musicals are Victor Borge and soprano Marylyn Mulvey in "Comedy with Music," "Side by Side by Sondheim," with Larry Kert, Hermione Gingold, Nancy Dussault and Georgia Brown, and "I Love My Wife," a four-character musical. That totals 14 shows with 28 players. Small wonder Equity's unemployment rate is astronomical.

In London, where ticket prices are almost within reason for the tax-riddled Britons, one-character plays also are in vogue.

Julie Harris drew cheers for her limited run in William Luce's Emily Dickinson portrait, "The Belle of Amherst," more of a "play" than most of the breed. Emlyn Williams, who can always do Charles Dickens or Dylan Thomas, has a new source, H. H. Munro, better known as "Saki" of the marvelous short stories. Rex Harrison has a one-man show on George Bernard Shaw. Robert Morley has been in Brighton to test a solo work about . . . Robert Morley. Audiences liked it but Morley doesn't and he's unlikely to continue. Ian McKellin, the Royal Shakespeare lead, has one he titles "Words, Words, Words."

In "Pictorial Smash," actor Mark Long has a vehicle that almost beats the odds. For the first 20 minutes of this one-man show, Lane doesn't appear at all. There are explosions, toppling rubble and performing props. Long finally crawls out to chat with his clothing and a passing bacon sandwich. His theme is today's anarchy.

In the British National Theater's small Cottesloe auditorium, actor Barry Collins plays a Russian soldier who explains his cannibalism in a monologue said to be based on fact. It is called "Judgment." It alternates with "Kemp's Jig," with Chris Harris as Shakespeare's bibulous Globe Theater associate.

That makes seven "plays" with seven actors or, for the two theater capitals, 21 "plays" and 35 players.

Two more mini-dramas are on the horizon. Burt Lancaster and Kirk Douglas will become middle-aged Tom Sawyer and Huck Finn in a new work scheduled for spring. Ian Richardson has been in Englans conferring with Cecil Beaton on a solo play drawn from beaton's prose, as sharp and chic as Beaton photographs.

It's interesting that adroit as they are, Harold Pinter, Neil Simon and Noel Coward never managed fewer than three characters (in "The Caretaker," "Star Spangled Girl" and "Design for Living"). Even Samuel Beckett's monologues and dialogues are limited to one act.

I'm getting uneasy about hthis retreat to mini-dramas and mono-dramas. And when they tell me it's art and not economics, I say it's spinach.