From Wiki:
The Roman legion (Latin: legiō, [ˈɫɛɡioː]), the largest military unit of the Roman army, comprised 4,200 infantry and 300 equites (cavalry) in the period of the Roman Republic (509 BC–27 BC) and 5,600 infantry and 200 auxilia in the period of the Roman Empire (27 BC – AD 476).
One of the times the Roman army was reorganized was after the war between Octavian and Antony. The century was reduced from 100 to 80, for operational reasons, and to increase the number of centurions (but the name was kept). As a consequence, legion size was reduced, cohort size was reduced. Duplicate legion numbers were eliminated. Overall number of legions was reduced to 28 (from 56), with the Praetorian Guard organized to serve as Rome’s police force and the imperial bodyguard. Auxiliaries were made permanent, drawn from conquered populations, and those numbered 28 legions. Enlistment lengths and terms were changed.
My guess is, the entire reorganization was the work of Octavian’s best friend and unfailing ally, Marcus Agrippa. Perhaps the only real changes from the so-called republic period into the formal Empire was the addition of the full-time chief executive and introduction of the the standing army with career soldiers, and the permanent existence of the Roman navy, with at least five major bases to sweep the seas of piracy.
Part of the reason for the endurance of the Empire was the peace through commerce model. Too bad about those plagues intercoursing up everything. Also too bad that no one tried to build an orderly system of succession until Diocletian, after the plagues and the rest of the Crisis of the 3rd Century.