Even in the course of a military campaign, the size of a Roman legion varied because, unlike the case of the Persian immortals, there wasn't always someone waiting in the wings to take over when a legionary (miles legionarius) was slain, taken prisoner or incapacitated in battle. Roman legions varied over time not only in size, but in number. In an article estimating population size in ancient Rome, Lorne H. Ward says that up to at least the time of the Second Punic War, a maximum of around 10% of the population would be mobilized in the case of a national emergency, which he says would be about 10,000 men or about two legions. Ward comments that in the early, close-to-annual border skirmishes, only the number of men in half a conventional legion might be deployed.
Early Composition of the Roman Legions
"The earliest Roman army consisted of a general levy which was raised from the aristocratic landowners .... based on the three tribes, each of which provided 1000 infantry.... Each of the three corps of 1000 comprised ten groups or centuries, corresponding to the ten curiae of each tribe."
p. 52 Cary and Scullard
The Roman armies (exercitus) were composed mainly of Roman legions from the time of the legendary reforms of King Servius Tullius [also see Mommsen], according to ancient historians Cary and Scullard. The name for the legions comes from the word for the levy (legio from a Latin verb for 'to choose' [legere]) that was made on the basis of wealth, in the new tribes Tullius is also supposed to have created. Each legion was to have 60 centuries of infantry. A century is literally 100 (elsewhere, you see century in the context of 100 years), so the legion would have originally had 6000 infantry men. There were also auxiliaries, cavalry, and non-comabatant hangers-on. In the time of the kings, there may have been 6 centuries of cavalry (equites) or Tullius may have increased the number of equestrian centuries from 6 to 18, which were divided into 60 units called turmae* (turma in the singular).
Increasing Number of Legions
When the Roman Republic started, with two consuls as leaders, each consul had command over two legions. These were numbered I-IV. The number of men, organization and selection methods changed over time. The tenth (X) was Julius Caesar's famous legion. It was also named Legio X Equestris. Later, when it was combined with soldiers from other legions, it became Legio X Gemina. By the time of the first Roman emperor, Augustus, there were already 28 legions, most of which were commanded by a senatorial legate. During the Imperial period, there was a core of 30 legions, according to military historian Adrian Goldsworthy.
Roman ancient historians Livy and Sallust mention that the Senate set the size of the Roman legion each year during the Republic, based on the situation and available men.
According to 21st century Roman military historian and former National Guard officer Jonathan Roth, two ancient historians of Rome, Polybius (an Hellenistic Greek) and Livy (from the Augustan era), describe two sizes for Roman legions of the Republican period. One size is for the standard Republican legion and the other, a special one for emergencies. The size of the standard legion was 4000 infantry and 200 cavalry. The size of the emergency legion was 5000 and 300. The historians admit of exceptions with legion size going as low as 3000 and as high as 6000, with cavalry ranging from 200-400.
"The tribunes in Rome, after administering the oath, fix for each legion a day and place at which the men are to present themselves without arms and then dismiss them. When they come to the rendezvous, they choose the youngest and poorest to form the velites; the next to them are made hastati; those in the prime of life principes; and the oldest of all triarii, these being the names among the Romans of the four classes in each legion distinct in age and equipment. They divide them so that the senior men known as triarii number six hundred, the principes twelve hundred, the hastati twelve hundred, the rest, consisting of the youngest, being velites. If the legion consists of more than four thousand men, they divide accordingly, except as regards the triarii, the number of whom is always the same."
~ Polybius VI.21
In the imperial legion, beginning with Augustus, the organization is thought to have been:
- 10 squads (contubernia - a tent group of generally 8 men) = a century, each commanded by a centurion = 80 men [note that the size of a century had diverged from its original, literal meaning of 100]
- 6 centuries = a cohort = 480 men
- 10 cohorts = a legion = 4800 men.
There is some evidence that in the first century the size of the first cohort was doubled:
"The question of the size of the legion is complicated by the indications that, at some point subsequent to the Augustan reform, the organization of the legion was altered by the introduction of a doubled first cohort.... The principal evidence for this reform comes from Pseudo-Hyginus and Vegetius, but in addition there are inscriptions listing discharged soldiers by cohort, which indicate that about twice as many men were discharged from the first cohort than from the others. The archaeological evidence is ambiguous... at most legionary camps the pattern of barracks suggests that the first cohort was of the same size as the other nine cohorts."
* M. Alexander Speidel ("Roman Army Pay Scales," by M. Alexander Speidel; The Journal of Roman Studies Vol. 82, (1992), pp. 87-106.) says the term turma was only used for the auxiliaries:
"Clua was a member of a squadron (turma) - a subdivision known only in the auxilia _ led by a certain Albius Pudens.' Although Clua named his unit simply by the colloquial expression equites Raetorum, we can be certain a cohors Raetorum equitata was meant, perhaps cohors VII Raetorum equitata, which is attested at Vindonissa during the mid-first century."
The Imperial Army Beyond the LegionsComplicating questions of the size of the Roman legion were the inclusion of men other than the fighters in the numbers given for the centuries. There were large numbers of slaves and civilian non-combatants (lixae), some armed, others not. Another complication is the likelihood of a double-sized first cohort beginning during the Principate. In addition to the legionaries, there were also auxiliaries who were mainly non-citizens, and a navy.
- "Roman Population, Territory, Tribe, City, and Army Size from the Republic's Founding to the Veientane War, 509 B.C.-400 B.C.," by Lorne H. Ward; The American Journal of Philology, Vol. 111, No. 1 (Spring, 1990), pp. 5-39
- A History of Rome, by M. Cary and H.H. Scullard; New York, 1975.
- "The Size and Organization of the Roman Imperial Legion," by Jonathan Roth; Historia: Zeitschrift für Alte Geschichte, Vol. 43, No. 3 (3rd Qtr., 1994), pp. 346-362
- How Rome Fell, by Adrian Goldsworthy; Yale University Press, 2009.