The bulk of the empire’s forces are not legionnaires but members of the Crimson Tide. The Tide is a loosely organized army that aggressively recruits from newly conquered lands (one can save his/her family from slavery by joining the Tide as Tide members are automatically granted citizenship).
In battle the new recruits are usually sent out as the first wave, most of them don’t live to see their third battle. Veteran Tide soldiers join the battle in the second wave, after the recruits have softened up the ranks of the enemy.
The Tide was founded to make up for the low numbers of the Legion and provide dispensable soldiers both for costly assaults and boring garrison duties. The Tide is led and funded by merchants and noblemen who try to leverage their support to gain as much power and economic advantage as possible in the newly conquered lands.