The men it the army at the time of Cannae all owned property so many of them probably did not live in the city itself. I could only find population numbers for later dates which make the put the Roman population arround 1,000,000 two hundered years later so it would have been smaller at the time of the Second Punic War. between 45,000 and 80,000 Romans were killed or captured at Cannae so even if the battle took place when Rome had 1,000,000 people and then 1 in 20, possibly more would have been lost ad Cannae. Of this population of 1,000,000 more then half of theose would have been non-citizens or Women. It is important to note that most of Romes allies who fought with them against Hannibal were not full citizens.