Genetics indicates that the ancestral African population that gave rise to modern Eurasians had an effective population size of about 14,500 individuals. Those who left Africa 50 kya were about 1860 in number. Numbers had not increased by the time Europeans and Asians split 23 kya, and consisted of about 1032 for Europe and 550 for Asia. It seems to have take a while to get the hang of the new continent.
From Evolution, by Douglas Futuyma.