Kiri Te Kanawa has been heard by more people than any other singer in the history of opera. An estimated 800 million were tuned in when she sang at the British royal wedding last year, not to mention a few million who have caught her, live or recorded, on other occasions. The number will grow by another 2,909 tonight when she makes her recital debut for a standing-room audience (with 100 people seated on the stage) in the Kennedy Center Concert Hall, and a few million more will be added Oct. 7 when her performance in the Met's "Der Rosenkavalier" is broadcast live on PBS. But the number that sticks in her mind is 800 million.

"You can't do anything much more frightening than that," she was saying a few days ago. "Someone figured out that I would have to do 25,000 performances at Covent Garden . . . no, it was more like 250,000 . . . to reach an audience of 800 million. I didn't think too deeply about it. When something is so enormously out of your control in a situation like that, I think the only thing to do is to go in blindfolded and just do it. If I had to think about it I suppose I would have been really stymied."

The only negative comments she heard were about her dress, a turquoise creation with matching hat perched forward on her head -- "very busy; a lot going on," as described by one fashion expert. According to Te Kanawa, "The dress was very beautiful. I was asking my husband the other day -- we talk on the telephone a lot -- should I ever wear that dress again? It has been seen."

She was in Manhattan, the day after her triumphal first performance in the Met's "Der Rosenkavalier," and the living room of her apartment near Lincoln Center was full of flowers showered on her by fans. "There are flowers in the kitchen, flowers in my bedroom, flowers everywhere," she was saying. "It took five of us to carry them over here last night, and I didn't get to bed until 4 in the morning. They're so pretty, I love having them around -- but it will be very sad when they start to die."

That was a Marschallin kind of reflection; her character in the Strauss opera (a role she has sung only once before, in Paris) is an aging member of the nobility in 18th-century Austria who is at the center of two of opera's most touching moments: a long, heartbreaking monologue on the ravages of time, sung as she sits alone with her mirror in Act I, and a trio in Act III during which she gives up her young lover, Octavian, to an appropriately youthful bride. Kiri Te Kanawa talks about the Marschallin sometimes as a real human being, sometimes as a challenge: "The Marschallin--as they say in America, she's some lady. We're just at the beginning of a long relationship."

At 38, Te Kanawa shows none of the ravages that she sings about so eloquently. Born to a European mother and a Maori father, she was raised in New Zealand but now lives in England, where her two children are in school. At home in her temporary apartment, her beauty does not glitter as it does under spotlights, but it radiates, quiet and (a quality rare among star performers) wholesome. She speaks softly, relaxing on a sofa and looking occasionally at the flowers, tokens of the kind of insane adulation that only sopranos (and an occasional tenor) seem to excite. That excitement does not seem to affect her; she is Kiri Te Kanawa's severest critic.

The Marschallin has more than an hour offstage between the end of Act I and her return halfway through Act III, and she uses some of that time "to look back at what went wrong and what can be done better," she says. "It's nice to have time in the middle of an opera. In 'Arabella' which she will be singing at the Met later in the season , I'm kept running around all the time; I have no time to think, and I wonder, 'My goodness, how am I going to get through to the end of this opera?' "

In spite of the critical acclaim, the long standing ovations, the rooms full of flowers, she is not satisfied with her Marschallin. "It's something I've just started to learn," she says. "Mastering it is something else, at least from my point of view, because I hold myself to very high standards. If it makes other people happy, fine, but I'm really never satisfied with anything."