Chromosomes consist of 22 pairs of autosomes and one pair of sex chromosomes (XY in male, XX in female), making a total of 46.
Abnormal numbers of chromosomes mean that the number of chromosomes is more or less than 46.  To be specific, the presence of an extra chromosome is called trisomy, and the presence of only one chromosome is called monosomy.
The abnormal numbers of chromosomes cause various problems, such as physical abnormalities, decreased intelligence, and poor growth of the fetus.