I've read that Rome had lost around 20% of its military eligible population during the early stages of the war. Losing battles that decisively would have likely broken any other country of that era and those types of losses would likely cripple modern nation-states even if they were in a total war mentality. What makes it even more confusing is that they not only survived these events but managed to turn the tide of the war and win it.
How did they pull this off?
Well first you need to look at the way that Roman society was structured, in modern era to loose 20% of your population would severily cripple your infrastructure, however the romans deployed thousands of slaves in order to keep afloat their economy and industry, another factor that comes to bear is that in pre-marius reforms, the military was composed of landowners, so even tho the romans had lost a lot of people, there where still a good amount pool of romans who did not signed up at the first go, also you got your latins and italians, then theres mercenaries, Rome would indeed have employed from pirates to greeks and africans.