So Harbinger was created around 1,000,000,000 BCE, Harbinger never would have been able to conquer the next powerful race on his own in the same time it takes for the reapers to conquer the protheans (which took centuries)
Let's say as a rough estimate that the Reapers come back every 50,000 years(which is fair since the reapers leave non spacefairing races , which could range from bronze age to the nuclear bomb in terms of current technology level)
Since the reapers harvest every conquered civilisation into a reaper
That's 1 billion / 50,000 = 20,000
The Reapers strength by humanity's arrival at the citadel is 20,000.
Now that number could probably vary, maybe destroyer class reapers are the lesser races of the cycle they wipe out, with the sovereign class reapers being the big dogs.
But if I'm correct, and the reapers process all races into a single new reaper, and every civilisation destroys two reapers each cycle. Then there would be no more reapers left in 500 million years
The current cycle destroys sovereign, the destroyer on earth and the one on Rannoch I think it is (3 if you count the one that the kelros kills on Tuchanka)
If you have enough war assets, two sovereign class reapers get destroyed in the fleet battle above Earth
And that's only in a few months of fighting, who knows how many the Protheans killed in the centuries that they had to fight, or the civilisations before that.
The only conclusion I can come to from that is that EACH major race is processed into a Sovereign class reaper.
So if this cycle was lost
Humanity, the Asari, Turians and Salarians would all become Sovereign Reapers
Then the Krogan, Elcor, Hanar, Quarians, and whoever else I've missed would become Destroyer Reapers.
Now I feel autistic.
-KL