Rights of the Roman soldier

[Tiberius Clodius Corvinus]

May be this helps on your question:
Ross Cowan in Osprey Warrior No. 71 cites Scheidel:

In times of relative peace a legion of 5,000 men probably suffered a decremental mortality rate of about 40 per cent over a 25-year service period (indicative of the endemic diseases in the Roman world), and a further 15 per cent through soldiers invalided out of service. Consequently the legion would require 280 recruits annually to maintain optimum strength.

You have to calculate further yourself ... 8)