The army with which Julius Caesar fought his campaigns in Gaul consisted of legions—four in the first instance, subsequently increased to ten—with auxiliaries attached.The Legion.
The word legio, which originally denoted a “muster” of citizens for infantry service in the force raised year by year for the early wars of Rome, came in due course to denote a “mixed brigade,” of infantry and cavalry—to which, by Caesar’s time, artillery also was added. A consul’s, or pro-consul’s, army consisted of two such legions. Tradition ascribes to one of the Kings, Servius Tullius, the introduction of a property qualification for the levy (dilectus): but, as military service became more and more distasteful to the Roman citizens, the property qualification was lowered, and when C. Marius enrolled legions in 104 b.c., at a time of grave disaster and impending invasion, he was glad to accept capite censi—citizens indeed, but poor men and even penniless, who looked to the service for their livelihood. Thus the professional soldier replaced the citizen soldier.
From the beginning of the Republic until the time of the 2nd Punic War (218 b.c.), the infantry establishment of the legion was about 4,200; thenceforward, until the time of Marius, it varied from 4,200 to 5,000, or even 6,000. In Caesar’s day the establishment appears to have been about 5,000; but the actual strength of his legions was rarely up to establishment, as may be inferred from B.G. v. 49, where he reckons two legions, with some cavalry, as 7,000 men.