The human genome reference sequence contains 24 distinct chromosomes: the 22 autosomes plus the X and Y sex chromosomes.
That number can seem surprising because most human body cells contain 46 chromosomes. The figure 46 counts two copies of each chromosome—one inherited from each parent. A reference sequence, however, represents one version of each distinct chromosome type, so it is organized as chromosomes 1 through 22, X, and Y.
The reference genome is not the complete DNA sequence of one individual. It is a standardized composite used as a coordinate system for comparing genomes, locating genes, and describing variants. Scientists continually update its assemblies as gaps are closed and regions of human diversity are represented more accurately.
Another common confusion is the number 23, which describes the number of chromosome pairs in typical human cells. The reference sequence counts chromosome types, not pairs; therefore its canonical count is 24.