At the time of Augustus, when Rome had a population of a million citizens and the the empire had a population of approximately 40-60 million people, how many Roman aristocrats were life-long members of the senate?
During Augustus' reign he drastically reduced the number of Senators from 900 to 600, by going through the list and pretty much picking the one's he preferred. The one's he preferred were the one's of high birth, free born and had property that was equal to or exceeded 1 million sesterces.
Good books if you're interested are: Society and Politics in Ancient Rome (1909) A history and description of Roman political institutions (1911) (both by Frank Frost Abbott)