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
The Senate as the State-Council of the Monarch
While the democratic king thus conceded to the community of the people at least a formal share in the sovereignty, it was by no means his intention to divide his authority with what had hitherto been the governing body, the college of senators. The senate of Caesar was to be--in a quite different way from the later senate of Augustus-- nothing but a supreme council of state, which he made use of for advising with him beforehand as to laws, and for the issuing of the more important administrative ordinances through it, or at least under its name--for cases in fact occurred where decrees of senate were issued, of which none of the senators recited as present at their preparation had any cognizance.
There were no material difficulties of form in reducing the senate to it original deliberative position, which it had overstepped more de facto than de jure; but in this case it was necessary to protect himself from practical resistance, for the Roman senate was as much the headquarters of the opposition to Caesar as the Attic Areopagus was of the opposition to Pericles. Chiefly for this reason the number of senators, which had hitherto amounted at most to six hundred in its normal condition(21) and had been greatly reduced by the recent crises, was raised by extraordinary supplement to nine hundred; and at the same time, to keep it at least up to this mark, the number of quaestors to be nominated annually, that is of members annually admitted to the senate, was raised from twenty to forty.(22)
21. Cf. IV. X. Abolition of the Censorial Supervision of the Senate
22. According to the probable calculation formerly assumed (iv. 113), this would yield an average aggregate number of from 1000 to 1200 senators.
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Reference address : https://www.ellopos.net/elpenor/rome/5-11-old-republic-new-monarchy.asp?pg=45