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
This phalanx was the army destined for the field, while a like force of troops was reckoned for the seniors who remained behind to defend the city. In this way the normal amount of the infantry came to 16,800 men, 80 centuries of the first division, 20 from each of the three following, and 28 from the last division--not taking into account the two centuries of substitutes or those of the workmen or the musicians. To all these fell to be added the cavalry, which consisted of 1800 horse; often when the army took the field, however, only the third part of the whole number was attached to it.
The normal amount of the Roman army of the first and second levy rose accordingly to close upon 20,000 men: which number must beyond doubt have corresponded on the whole to the effective strength of the Roman population capable of arms, as it stood at the time when this new organization was introduced. As the population increased the number of centuries was not augmented, but the several divisions were strengthened by persons added, without altogether losing sight, however, of the fundamental number. Indeed the Roman corporations in general, closed as to numbers, very frequently evaded the limit imposed upon them by admitting supernumerary members.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=17