Pizza Hut Inc. said yesterday it is hiring 20,000 workers--increasing its employment by nearly one-third--to staff the chain's 3,800 restaurants to sell a new six-inch luncheon pizza. The company said it is expecting the new product to produce a "dramatic" increase in sales.

The company has already hired most of the 500 new employes it needs in its 50 Washington-area restaurants, according to Dennis Bowers, Pizza Hut's Washington district manager. Most of the openings were part-time positions, paying the minimum wage, he said.

The luncheon pizza was introduced in this area Jan. 24; national introduction is set for Monday.

The new pizza sells for under $2 and is guaranteed to be ready for customers within five minutes of the order.

"Essentially what we are doing is entering the fast-food lunch market," said a spokeswoman for the Wichita, Kans.-based Pizza Hut chain, a subsidiary of Pepsico Inc. She said that the additional employes are needed because Pizza Hut restaurants, while doing a strong dinner business, have in the past been much less busy at lunch.

Hiring of the 20,000 employes, both part-time and full-time, has already begun and will continue for the next 90 days, the spokeswoman said. The positions will be both part-time and full-time, the equivalent of 10,000 full-time positions in all, according to the company. The number of employes hired by each restaurant will vary, but the company expects the average to be four to seven new workers per restaurant.