Legionary Training

[Nathan Ross]

I still believe that there was no official reinforcement of legions until the institution of a standing army under Augustus. Before this, legions were raised together on the citizen militia principle, and discharged en masse. Reinforcing them with fresh troops would have messed up the discharge dates, and may have been illegal anyway.

So some legions did become very understrength, especially during the civil wars. At Pharsalus Caesar got around this by combining several of his depleted veteran legions into temporary mixed units.

There's a previous thread about this very topic:

Legion%20Reinforcements]