The Legions were composed of Roman citizens mainly drawn from Italy at first with a lot more coming from the Iberia provinces, Gallia and others by the end of the first century AD.
During approximately the first half of the Principate the legio was divided into ten cohortes plus a small number of mounted men known as the equites legionis. This cavalry unit was mainly devoted to communication, logistics and other non-combat activities and so the Roman Army of the Principate relied on auxiliary forces to provide mounted forces.
Each cohors was composed of six centuriae and each centuria comprised 10 contubernia of eight men each that fought, slept and trained together forming strong bonds within them. The theoretical strength of a Roman legion was then 8 x 10 x 6 x 10 = 4.800 men
If you’re wondering why a unit of 80 men (10 contubernia of 8 men each) was called centuria (latin word for a hundred), the answer lies in two additional men allocated to each contubernium for non fighting duties, raising the total number to 100 (20 additional men per centuria)
At some time during the Principate, probably at the beginning of the Flavian era, the first cohors or cohors prima was reorganized to include five centuriae of 20 contubernia each and including the most veteran legionaries
In battle the Legio typically formed in what was called triplex acies with four cohortes in front and then two lines of three cohortes each in a kind of checker board pattern, even if it could use a two line deployment at times.