What if a human has more or less then chromosomes?

Most of the time, the birth is aborted or the fetus dies. The body simply can't develop without the full sets of chromosomes. However, it depends on which chromosome is missing. For example, if one of the sex chromosomes is missing, leaving just one X, then you end up with Turner Syndrome. Having a third 13th chromosome can cause Patau syndrome. An extra chromosome on chromosome 21 may give rise to Down Syndrome. So while most of them end up dying, there are some exceptions. Hope that helps.