My guess, twenty percent.
Give or take a few wounds.
5.56mm
About 12 percent attrition over the 25 year period of enlistment, which could easily be at least as good as the rest of the population from which the legions were drawn. Getting a farm at discharge was a big boon for the veterans. During the so-called republic, patriotic volunteers risked everything to save the city-state, and found that whatever home they'd had before had been taken over by some a-hole from one of the three dozen extended families who owned most of Italy. IF the soldier was lucky, he'd be able to find the family he'd left behind and they wouldn't have been enslaved by those very same a-holes.