Short answers, pulled from the story.
A Roman legion in the Imperial period typically fielded 5,120 legionaries, organized into ten cohorts. When combined with an equal number of auxiliary troops, the total force available to a legion commander could reach around 11,000 men. The first cohort was almost double-strength, at 800 men, while the remaining nine cohorts each held 480.
A rank-and-file Roman legionary received 225 denarii a year until the emperor Domitian raised it to 300. Septimius Severus later increased pay to 500 denarii a year. Upon completing 25 or more years of service, veterans received a discharge bonus of 3,000 denarii from the time of Augustus, a figure Caracalla later raised to 5,000 denarii.
From 104 BC onward, each Roman legion carried an aquila, an eagle standard, as its symbol. Losing the aquila in battle was considered the greatest dishonor a legion could endure and often led to the unit's disbanding. The officer who bore it, the aquilifer, was a chosen veteran paid twice the base wage.
Decimation was a punishment reserved for units that mutinied, deserted, or showed dereliction of duty. One man in every ten, selected by lot, was beaten to death, usually by the other nine members of his unit. The survivors were forced to live outside the fortified camp and in some cases had to renew the military oath.
Legio XVII, Legio XVIII, and Legio XIX were founded by Augustus around 41 BC and were destroyed by a Germanic alliance led by Arminius on the 9th of September, AD 9, in the Varian Disaster. The Romans never raised these three numbered legions again. The disaster reduced the total number of legions under Augustus from 28 to 25.
The Roman legion system outlasted the fall of the Western Roman Empire and continued within the Eastern Roman Empire until the 7th century. Emperor Heraclius then reformed the military into the Theme system to address the increasing need for soldiers. Legio V Macedonica, founded in 43 BC, is recorded as still active in Egypt during the Arab conquest of 637 AD.