@monkaS
If we assume that f(x) = k*x^a, where f(x) is the number of soldiers needed to defeat 100 orcs, then, given the conditions cited in the chapter (f(2x) = 3*f(x), f(100) = 100 and f(200) = 300), k ~ 0.07 and a ~ 1.58.
f(1000) ~3846
f(3000) ~ 21937
BTW, a single orc is a pussy, defeatable by 0.07 men. Even the red orc can be solo'd by a guy as strong as 0.39 average soldiers, as long as it's one on one.