Based on discharge inscriptions, the Roman army retired around 150 veterans per 5,000 soldiers, representing about 3% of its troops each year. The military would also lose another 2% to natural rates of adult mortality. However, since somewhat less than half of all soldiers lived to complete their 25 years of service, the actual rate of mortality among soldiers was likely higher, at around 3%-4%.
This means that each year the military lost around 6%-7% of its manpower to retirement and mortality. With an imperial army size of around 400,000 legionaries and auxiliaries, the Roman military required 24,000–28,000 new recruits simply to maintain its size.
Each decade, this massive and complex military machinery would need to recruit, train, and churn out 240,000–280,000 new soldiers to defend the empire, essentially replacing most of the soldiery each decade.