User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-50.80.96.33-20130124010235/@comment-4967682-20130124012651

There are, being generous, a little over a million Reaper ships, counting their non-sentient drones (Oculi, transports, processors) and Collectors. They're a billion years old and gain one capital Reaper and a dozen Destroyers every 50,000 year cycle, meaning 20,000 capital Reapers and 240,000 Destroyers. Add all of the Oculi (they must be common in comparison to the Reaper's dreadnought and frigate equivalents) and other non-sentient ships and generously they clock in at a little over a million. There are possibly hundreds of billions of Forerunner ships. They're not on the same level.

Like I said, even if the Reapers exploit every single "I win" button, there's too many Forerunners to indoctrinate, too many planets to RKV into, and too many solar systems to make simply building millions of bombs an option (it took them what, a century to make the mass relay system?).