Cytologists had been studying chromosome behavior since the late 19th century. Why did it take until 1956 to figure out the correct human chromosome number, thanks to Joe Hin Tjio and Albert Levan?

Each human cell contains 23 pairs of chromosomes that carry DNA ... Female (XX) mammals inherit one X chromosome from each parent, but males (XY) receive an X from their mother and a Y sex ...

explains that humans have 23 pairs of chromosomes: 22 autosomes and one pair of sex chromosomes—XX in females and XY in males ...