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
Population - Agriculture and the Rearing of Cattle
Gaul was, according to the reports of the ancients, comparatively well peopled. Certain statements lead us to infer that in the Belgic districts there were some 200 persons to the square mile-- a proportion such as nearly holds at present for Wales and for Livonia--in the Helvetic canton about 245;(11) it is probable that in the districts which were more cultivated than the Belgic and less mountainous than the Helvetian, as among the Bituriges, Arverni, Haedui, the number rose still higher.
11. The first levy of the Belgic cantons exclusive of the Remi, that is, of the country between the Seine and the Scheldt and eastward as far as the vicinity of Rheims and Andernach, from 9000 to 10,000 square miles, is reckoned at about 300,000 men; in accordance with which, if we regard the proportion of the first levy to the whole men capable of bearing arms specified for the Bellovaci as holding good generally, the number of the Belgae capable of bearing arms would amount to 500,000 and the whole population accordingly to at least 2,000,000. The Helvetii with the adjoining peoples numbered before their migration 336,000; if we assume that they were at that time already dislodged from the right bank of the Rhine, their territory may be estimated at nearly 1350 square miles. Whether the serfs are included in this, we can the less determine, as we do not know the form which slavery assumed amongst the Celts; what Caesar relates (i. 4) as to the slaves, clients, and debtors of Orgetorix tells rather in favour of, than against, their being included.
That, moreover, every such attempt to make up by combinations for the statistical basis, in which ancient history is especially deficient, must be received with due caution, will be at once apprehended by the intelligent reader, while he will not absolutely reject it on that account.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=15