In 1912 it was suggested that the normal number of human chromosomes was forty-seven; This theory was revised in 1923, when it was decided that in fact humans have forty-eight chromosomes, a notion that stood for a third of a century.

No, Forty-six.

In 1956 two researchers proved that the actual human chromosome number is forty-six. J. Hin Tjio and Albert Levan studied human embryo cells. They took painstakingly careful photographs of many cells to prove that humans have forty-six chromosomes per cell (except for germ cells, which have twenty-three chromosomes per cell). In 1959 Jerome Lejeune reported a new finding after studying mongolism, now called Down's Syndrome. Lejeune found that children with mongolism had forty-seven chromosomes instead of forty-six.

Source:

Edgardo Macorini, ed. The History of Science & Technology: A Narrative Chronology (New York: Facts-on-File, 1988)