What is DNA?

In this video, Robin states that we each have 24 chromosomes. Humans have 23 pairs of chromosomes; 22 pairs are autosomal or body chromosomes, one pair is the sex chromosomes. Because the sex chromosomes are sometimes discussed differently, they can be throught of as two seperate chromosomes. Thus, Robin is referring to the 22 pairs, plus the two sex chromosomes for a total of 24. 
For more info, read here about human chromosome numbers.