Livy reports the existence of a "legio" under Romulus and several under Tullus Hostilius. But its real creation dates from the so-called reform of Servius Tullius:a legion then comprised 4,000 men divided into centuries.
In the early days of ancient Rome, the Roman warrior was very close to the hoplites of the Hellenic world, both in equipment and in tactics. (webmaster's comment)
During the Punic Wars, the army was reorganized:the legion was then made up of 30 maniple, each maniple comprised 2 centuries, but the centuries now comprised 60-80 men, making a total of 4,250 men. These forces are divided into three legionary ranks and the light troops:in the 1st rank in order of battle, we find the hastati - the youngest, who take the shock, 1,200 men - in the second rank the principles - of mature age, 1,200 h - and in the 3rd the triarii - the oldest, 600 h - and between these ranks or in front for skirmishes are interspersed the velites - light infantry, poorer, 1,200 h. And each legion is joined by a cavalry unit of 10 turmes of 3 decuries, i.e. 300 cavalry.
At the end of the 2nd century BC. J.-C., Marius transforms the Roman army into a professional army. The legion then comprises 10 cohorts of 3 handles (one handle of each rank:triarii, principles, hastati) and 2 centuries per handle (of 100 h), or about 6000 men (figure probably rarely reached). However, the equipment no longer being paid for by the legionnaire, but provided by the consul who raised the legion, it is standardized and becomes the same for the three ranks; moreover, the legionnaires being at that time volunteers, there is also no longer any age difference between the centuries, which are homogeneous and of the same value.
During the Early Empire, the overall strength of the Roman army was 33 legions from Septimius Severus (who created Legiones I, II and III Parthica), i.e. between 165,000 and 198,000 men. The vast majority of the legions are confined to the borders, thus following the Augustan defense system. Only the Legio VII Gemina, isolated in Tarraconaise and the II Parthica in Alba, Italy, which together constitute the only strategic reserves of the Empire, escape this rule.
Under the Lower Empire, the numbers changed. Diocletian the first modifies the number of soldiers of each legion. From now on, in addition to the classic legions (32 under Diocletian, because the VI Ferrata has disappeared), there are legions of around 1000 men. The essential reform, however, belongs to Constantine. He creates a powerful maneuver army of a good level, the comitatensis. This contains legions, with changing numbers, but also cavalry wings, or quite simply, numeri, troops without a given number. In this respect the Notitia dignitatum gives interesting information, and among others the respectable figure of 174 legions... Most of them do not have a large workforce. Other troops, including legions of lesser value, are confined to the defense of the borders of the Empire, the limitanei. The legion is already no longer the queen of the battlefields, nor the basic unit of the army.