Then you would guess wrong, at least in the ‘top 6’ schools part.
Whether you meant the top 6 public schools (TAG in Dallas, BASIS in AZ, TJ in VA, Gwinnet in GA, etc.?) or private (Exeter, Andover, College Prep in CA, Horace Mann in NY, Castilleja, Cate, Hotchkiss, SFU, etc?), I would estimate that 2/3 of the students would meet the criteria for interview. I can also tell you that at least half of the students in my D’s classes- at a school that does not crack the top 25 list- would qualify.
As you know, the minimum requirements for admission are very straightforward, and heavily based on standardized testing. There were about 23K AP National Scholars in 2013- ie, with a 4 or better in 8 or more APs, and average score of at least 4. You can proxy that most or all of the scholars would be able to muster 3/5 scores of 5, which is what Oxford/Cambridge require. So most of those scholars would likely meet the basic criteria.
In fact, I know something about most of the US students currently at Oxford (enrolled, not study abroad), and the schools they come from are remarkably ordinary. Moreover, most of them were not the ‘superstars’ of their secondary schools, mostly b/c they are more asymmetrical than most high school ‘superstars’.
Oxford would say that there is no limit on how many students could get interviews: they go for the best. However, given that the numbers are pretty steady over time, it would appear that they at least informally have a percentage basis, as admissions are done by department and college, not by the university overall.
So, in practice, I would think 150 is not far off, perhaps a bit on the high side.