When Primus created the Transformers, their population "expanded to fill the planet". Because Cybertron is about the size of Saturn, it's not unreasonable to expect twenty billion or more Autobots. Then we have the Great War. In this war, it is widely known that both Autobot and Decepticon armies and population take massive, innumerable casualties and that in the end very few Transformers are left before the Exodus.
Then when we're back to modern Transformer movies by Micheal Bay, and there are fewer than fifty Autobots that survive and reach Earth, where their leader Optimus Prime called them to go. In years their number is halved, and then later halved again, due to attacks by TRF, Cemetery Wind, and Lockdown. Why? There are several ideas I have about this.
- Autobots got slaughtered and killed when there was an accident (like spaceships got a virus and powered down)
- The Great War killed almost all the Autobots
- A lot of Autobots didn't answer Prime's call or couldn't
We see only a few hundred or so Decepticons in the battles of Egypt and Chicago, not even in the thousands. In each new Transformers movie, I was always expecting a "return of the Autobots", where the Autobots would be somewhat equal to the Decepticons in number.