Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Falling Off in the Population
With every allowance for the inequality in the political and economic circumstances of the different districts and for the comparatively flourishing condition of several of them, the retrogression is yet on the whole unmistakeable, and it is confirmed by the most indisputable testimonies as to the general condition of Italy. Cato and Polybius agree in stating that Italy was at the end of the sixth century far weaker in population than at the end of the fifth, and was no longer able to furnish armies so large as in the first Punic war. The increasing difficulty of the levy, the necessity of lowering the qualification for service in the legions, and the complaints of the allies as to the magnitude of the contingents to be furnished by them, confirm these statements; and, in the case of the Roman burgesses, the numbers tell the same tale.
In 502, shortly after the expedition of Regulus to Africa, they amounted to 298,000 men capable of bearing arms; thirty years later, shortly before the commencement of the Hannibalic war (534), they had fallen off to 270,000, or about a tenth, and again twenty years after that, shortly before the end of the same war (550), to 214,000, or about a fourth; and a generation afterwards--during which no extraordinary losses occurred, but the institution of the great burgess-colonies in the plain of northern Italy in particular occasioned a perceptible and exceptional increase --the numbers of the burgesses had hardly again reached the point at which they stood at the commencement of this period. If we had similar statements regarding the Italian population generally, they would beyond all doubt exhibit a deficit relatively still more considerable.
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Reference address : https://www.ellopos.net/elpenor/rome/3-12-management-land-capital.asp?pg=48