Also for anyone doubting Amon further, here are a couple more explanations. If this seems directed and out-of-context, it is.
Context here.
"By "after the Overmind's death" I mean after Amon returned from his imprisonment during Legacy of the Void. He HAD his own swarm to lead during the campaign.
Also, he was going to control everyone before destroying everything except his Hybrid. Then, yes, he would start anew. That does not lessen the fact that he was indeed going to control everyone at some point. The Overmind HIMSELF states "and my Zerg will be saves to the Hybrid. All will bow before the power of the Fallen One."
Only after he destroyed the Protoss in their last stand on the Shadowy World would he obliterate the universe and remake it in his image. Even though he could destroy them immediately he wanted to break them. He has to stroke his ego, and he does that pretty much all the time. Using his mental abilities to gloat to Artanis multiple times instead of trying to do meaningful damage, keeping people alive until he can break them, not destroying people/the universe until he can establish his utter dominance over it, the list goes on.
The number count of the Zerg should be obvious. The current human population is nearly 8 billion. It will progress to 10 billion before we hit our limit of food. The Terrans expanded to hundreds of worlds, and that is ONLY the Terrans. The Earth-humans are a completely separate branch of humanity. Terrans alone should be in the high billions to low trillions.
The Zerg are a near-literal flood of teeth and claws, and coat the surfaces of planets in their forces. It would take many MANY more than 10,000,000,000 Zerg to dominate a planet by numbers alone, as humans, surface area speaking, are few and far between. The Swarm could dominate hundreds of worlds at once to the point that significant portions of the galaxy are deemed "Zerg space." The Bone Trench on char is almost completely coated in the bones of Zerg. The number needed to fill that much surface area is astronomical. And all that is assuming that the Zerg are human-like in their conquering. Earth, one planet, houses over 20 quintillion living animals. While most are small, the Zerg are an all-consuming flood of biomass and should easily be able to rival these numbers when spread across their space. While there is no specific number for the Zerg and Terrans, to call their numbers less than quadrillions/quintillions is nothing more than downplay.
We don't need an absolute canon number, we can use canon material and apply it to the logical applications of numbers-based interstellar domination."