Originally Posted by
Yukitsu
The highest I've ever seen for Rome was 60 million people living within their borders, only about 10 million of them living in Italy. Officially, the Roman Empire seems to have had about 450,000 troops. Given this was at a period of relative peace though, it seems likely that if an adversary arose at that time, the Romans could have raised more.
I'm also trying to figure what the percentage of that population was slaves or conquered peoples that could not qualify for the army. I'm not exactly certain whether or not they could have raised more if those slaves were free or if having slaves required more military presence by such a huge margin that it levels out or favours the army. Given how many slaves were in the city of Rome itself which wasn't defended by the legions directly it seems likely that full time military were not used to counter slave populations.
Those numbers may also fail to include the Vigiles which were the professional police of Rome.
By percent though, the Roman Empire had fewer men in the legions than they had during the height of the Republic. At the height of Roman military power in the Republic they managed over 700,000 legionaries with a population less than half that of the Empire. That was after the Marian reforms where a legionary was a professional soldier as compared to the Hastati civil soldier.