In the beginning a Roman legion was the whole of the army:
The latin word
legio means army and that was what the army was, initially.
But a larger army than one single legion became needed, when Rome grew and found evermore powerful enemies.
The smallest unit was the century, commanded by a centurion. The centurion was an experienced soldier who had proven to be brave but not reckless. His second in command was the optio. His place was in the back of the century, to keep the men in place.
A century consisted of 60 legionnaires of the same kind. I.e. All hastati or principii or triarii. This number was not really fixed and could be expanded to 120 in times of crisis. A century of triarii consisted of only 30 legionnaires. This number was never changed.
Important figure in the century was the signifer, who bore the the standard of the century. Losing the standard in a battle would mean loosing the honour of the unit.
The right wing gave more trouble. Here Agrippa, whose age and strength made him fearless, seeing that things were going better in all parts of the field than with him, seized standards from the standard-bearers and advanced with them himself, some he even began to throw amongst the masses of the enemy. Roused at the fear and disgrace of losing them, his men made a fresh charge on the enemy, and in all directions the Romans were equally successful.
A legion consisted of ten manipels of each type.