Board Thread:Theories and Speculation/@comment-3568954-20150602045122/@comment-5391752-20150602091128

I was also surprised that there could be around 100,000 shinobi going to war, considering their short life expectancy and how few apply for chunin exams. But here's a few reasons it might be possible. - Remember the start of the series: a genin was basically an odd-job labourer who could double as militia. - So many of the 100,000 may be part-time genin, working on contract basis and having another job elsewhere. So they were quickly enlisted/mobilized upon declaration of war. - Each Hidden Village might have contracts with nearby villages/settlements which have their own militia; all shinobi from one country normally fight under the same headband. - Smaller HV's from outside the Big 5 may also have contributed numbers if they were allied. - There are other career progression paths other than the Team X & chunin exams route we're familiar with. One could be inducted into Anbu, or into support work, or train with their clan.

By the way, the population in late feudal Japan was more than 10 million. It's feasible to raise an army of 100,000 at need.