Sgt Stryker wrote...
If the Reapers only need a few million humans to build a new Reaper, can't they just land and capture a couple of cities or a continent, and bombard the rest of the planet into submission?
No, 2 reasons:
A) They'd need to build more to replace Sovereign and any other Reapers that might be destroyed.

Getting only 1 reaper per cycle would be horribly inefficient (in fact didn't they confirm that the smaller reapers are made from ''lesser'' races, so this cycle Humanity would become a Harbinger-esque capital ship and maybe if they can scrape together enough Salarian/ Turian/ Hanar they'd build a few of the smaller ones).