From Live Science:
The first century B.C. was one of the most culturally rich in the history of the Roman Empire — the age of Cicero, Caesar and Virgil. But as much as historians know about the great figures of this period of Ancient Rome, they know very little about some basic facts, such as the population size of the late Roman Empire.
Now, a group of historians has used caches of buried coins to provide an answer to this question.
During the Republican period of Rome (about the fifth to the first centuries B.C), adult male citizens of Rome could be taxed and conscribed into the army and were also given the right to vote. To keep track of this section of the population (and their taxable assets), the Roman state conducted periodic censuses.Read more ....