HOW OFTEN do you get to see a perfect 10? On Saturday, you can see not one but two competing along with six 9s in a beauty of a contest.

We are, of course, talking about men (on horseback), and the 1985 International All- Star Polo Benefit Saturday at the Potomac Polo Club.

Every polo player registered with the U.S. Polo Association is ranked on a scale from -1, the lowest, to 10. There are only three "10- goalers" in the world, and two of them, Gonzalo Pieres and Ernesto Trotz of Argentina, will ride Saturday. The other players all carry 9s -- in a sport where only a handful of participants are ever rated above 6. They are Red Armour and Tommy Wayman of the United States; Hector Crotto, Christian Laprida and Alfonso Pieres of Argentina; and Antonio Herrera of Mexico.

While the players are obviously special, that doesn't mean the spectators need any special expertise to appreciate the game. At Potomac, announcer Lew Potter keeps the crowd filled in like a TV commentator at the Super Bowl, constantly explaining the rules, providing anecdotes about the players and describing the action.

And there's no shortage of action in a sport that dates back some 2,500 years to Central Asia, where a primitive caliph on horseback decapitated his enemy to create the first polo ball. There's nothing stylized or coy here: The field is filled with the sound of pounding hooves, the shouts of players and the crack of mallet against ball. It's a contest of speed, collision, combat. It's also the union of man and beast, the man acting and reacting, trusting and depending on a thousand-pound animal with a mind of its own. An old polo saying puts it this way: Behind every winning shot is a pony who saw it first.

Last year, organizers say, more than 10,000 people came to Potomac to see for themselves, some from vantage points within 10 feet of the sidelines, others from mid-field bleachers. (Besides the on-field action, there are tailgate picnics, an art show, boutiques and refreshments for sale.)

Polo play is relatively easy to understand. Four players (Nos. 1 and 2 on offense, Nos. 3 and 4 on defense) from each of the two opposing teams compete on a field 160 by 300 yards. Aboard horses racing and wheeling in a kaleidoscope of motion, riders lean virtually perpendicular to their mounts, using a 4 1/2-foot mallet to strike a 3 1/2-plastic ball the size of a softball. The idea is to knock it through the opponents' goal posts at the end of the field.

The game is so demanding that the horses (some worth as much as $25,000) rarely play more than one chukker in a row. There are six chukkers, or periods of play, in an outdoor polo game, each lasting 71/2 minutes. (After resting for a period or so, the horses come back for more.) Total game time, incling breaks, is about 11/2 hours. HORSE PLAY

Gates for the 5th Annual International All- Star Benefit open Saturday at 1; the match begins at 3:30. General admission is $5 per person; children under two, free. Proceeds benefit the Research Foundation of the Washington Hospital Center for Spinal Cord Injury.

If you want to catch further action after that, there are matches every Sunday from June 2 through October 6 at two different sites.

* At the Potomac Polo Club: 4 p.m.; general admission $3 per person, children under 12 free.

* At the Lincoln Mall Polo Field on the Mall: 3 p.m.; free. Also sponsored by the Potomac Polo Club.

In addition, from June through October Potomac Polo has scrimmages or matches Friday evenings in the outdoor arena at Potomac. Starting time and admission charge may vary. For further information, call Linda Bellino, 202/363-4367. GETTING THERE -- To get to the Potomac Polo Club: From the Beltway take River Road 11 miles to the T junction with Seneca Road; turn left on River Road for 41/2 miles, then right on Hughes Road and follow signs to the match. RIDE ON

If you really get hooked on polo and want to take your place on the field, stride for stride, knee to knee with the masters, the club also offers lessons. A new beginner course ($157.50 for five lessons) starts each Saturday in June. All necessary equipment and mounts supplied. For information, call Beverly Vaughn, 301/972-7493