score:3
I found two estimations about the number of slaves for the 2nd and 1st centuries BC.
From these numbers, we deduce that roughly 60% of the population were free. Halve this proportion to exclude women, further exclude children, and the proportion drops to 20-25% of free men (I have no precise idea for the proportion of children). Not all of them were citizens, but at least we have an upper bound.
[1]: Italian Manpower, 225 BC -- AD 14, Oxford, 1971
[2]: Servus. Rome et l'esclavage sous la RΓ©publique, Rome - Paris, 1987