The Washington Opera, fresh from a season in which it sold practically every ticket, will add 11 performances next year for a total of 57, general director Martin Feinstein announced yesterday.

He described it as "the longest operatic season ever offered at the Kennedy Center," and promised longer season to come. "I am no longer terrified, as I was when I took this job," he said. "I am now guardedly optimistic."

The session will open in the Opera House Nov. 7 with "La Boheme" (directed by Gian Carlo Menotti) and will run continuously until Jan. 16, 1982, concluding with "The Barber of Seville" in the Terrace Theater. The company will move upstairs to the smaller house, switching from grand opera to chamber opera, at the beginning of December, after giving five performances of each of its three Opera House productions -- one more performance per production than last season.

Between the bread-and-butter standbys that open and close the season, there will be two unusual operas, both in the Terrace. Igor Stravinsky's "The Rake's Progress" will be produced to mark the 30th anniversary of its composition, and the season's froth quota will be filled with a double feature: Gilbert and Sullivan's familiar "Trail by Jury," and "Monsieur Choufleuri restera chez luti le 24 janvier 1833," which Jacques Offenbach produced in 1861 in collaboration with the Count de Morny, an illegitimate son of Napoleon II.

More standard fare in the Opera House will be Verdi's "Macbeth," Mozart's "Magic Flute" -- with Max Rudolf conducting an English adaptation by Andrew Porter -- and a new production of Donizetti's "L'Elisir d'Amore."