What doctors and teachers call "Klinefelters' syndrome," was originally known as "Seminiferous Tubule Dysgenesis" and it was diagnosed by biopsy of the testes. But since STD is so hard to say it was abbreviated to "Klinefelters' syndrome." Then in 1959 when 1 man who had KS had his chromosomes counted, and he alone was found to be XXY, the karyotype was incorrectly called KS, or the terms were used interchangeably as if they were the same, but they are not.
Klinefelters' syndrome is a specific set of symptoms found in SOME XXY men. As most XXY men are never diagnosed at all, we can't say for certain how common KS actually is. But we can say with some degree of accuracy, that the XXY karyotype is found between 1:50
0 to 1:10
00 live male births. It will never be an exact number as XXY is a random event.
I was diagnosed with KS in 1976. I have many videos on YouTube about all manner of aspects of being XXY, and being treated for KS.