Short answers, pulled from the story.
The Roman army suffered a crushing defeat at the Battle of the Allia in 387 BC which forced a complete reorganization of military forces. This disaster led to the implementation of the Camillan system that divided earliest legions into three lines based on social class and wealth.
Polybius wrote his account around 150 BC describing an army shaped by the Punic Wars fifty years prior where each century contained roughly one hundred men. Equipment was self-provided with bronze helmets shields swords and heavy javelins called pila while triarii veterans fought as hoplites using Greek clipei and gladii if spears broke.
A typical Imperial legion held 5,280 men plus 120 auxiliaries split into ten cohorts after Augustus reduced the total number of legions to twenty-eight. Nine cohorts contained 480 men each while the first cohort doubled in strength to 800 men.
Legion V Macedonica created in 43 BC survived until the Arab conquest of Egypt in 637 AD. Many legions founded before 40 BC remained active until at least the fifth century AD but this specific unit persisted through the Islamic conquest of 637 AD.
Legionaries received 225 denarii annually until Domitian raised it to 300 denarii then Septimius Severus increased it further to 500 denarii. Veterans earned praemia benefits including 3,000 denarii or farmland plots after twenty-five years service and Caracalla later raised these benefits to 5,000 denarii per veteran completing their term.