The most surprising statistic in Wilt Chamberlain's new autobiography, A View From Above, has nothing to do with basketball. The former hoopster extraordinaire claims he's had sexual encounters with 20,000 different women. ''It's probably more,'' says Chamberlain, 56. ''I got the number by going through date books I've kept over the years since 1969, counting the names on a month-to-month basis and coming up with a prorated figure.'' That's more than 500 different encounters a year for the last 40 years or, as Chamberlain breaks it down in his book, ''1.2 women a day, every day since I was 15 years old.'' But Wilt denies that this kissing and telling is sexist braggadocio. ''They were never conquests. They were just what they were encounters. Something that we both took something from.'' Also, Chamberlain assures his readers that none of the women as far as he knows were married, because ''even as a single man, infidelity has no place in my life.'' In any case, he doesn't see his sexual scorecard as anything special: ''It was much harder to play 48 minutes a game for an entire season.'' Maybe so, but where did he find the time for basketball?