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
If we reckon as a minimum 10,000 full hides, this would imply a superficies of 190 square miles of arable land; and on this calculation, if we make a very moderate allowance for pasture, the space occupied by houses, and ground not capable of culture, the territory, at the period when this reform was carried out, must have had at least an extent of 420 square miles, probably an extent still more considerable.
If we follow tradition, we must assume a number of 84,000 burgesses who were freeholders and capable of bearing arms; for such, we are told, were the numbers ascertained by Servius at the first census. A glance at the map, however, shows that this number must be fabulous; it is not even a genuine tradition, but a conjectural calculation, by which the 16,800 capable of bearing arms who constituted the normal strength of the infantry appeared to yield, on an average of five persons to each family, the number of 84,000 burgesses, and this number was confounded with that of those capable of bearing arms.
But even according to the more moderate estimates laid down above, with a territory of some 16,000 hides containing a population of nearly 20,000 capable of bearing arms and at least three times that number of women, children, and old men, persons who had no land, and slaves, it is necessary to assume not merely that the region between the Tiber and Anio had been acquired, but that the Alban territory had also been conquered, before the Servian constitution was established; a result with which tradition agrees. What were the numerical proportions of patricians and plebeians originally in the army, cannot be ascertained.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=22