Most of it is trial & error. There isn't a guide for exact exposures that I know of, because you cannot just automatically account for where you are in the world. It would have to account for focal length, temperature, location on the planet, local light pollution(s), sky glow, and on and on and on. So most people take an exposure of a good length and then adjust from there based on trial & error.
When I started using my tracker, I started with 1 minute. Then 2 minutes. Then 4 minutes. I wanted to see what each "stop" of time would result in exposure wise for my location and particular settings (F2, ISO 400). Those three images told me what I needed to know about ISO and shutter speed stops in relation to the aperture wide open (which I was not going to change obviously).
The sky is not black. That's your first assumption mistake. You're shooting through an atmosphere that is far from 100% clear. And the space out there, again, is not black. There is dark space, but there's a lot of stuff out there and surrounding objects you're shooting.
People take the same shot over and over, for the purpose of stacking. Stacking is for reducing noise and hot pixels, keeping only the consistent light captured which results in keeping what you want, and reducing what's not supposed to be there. It's not for enhancing information, usually, it's for cleaning up, reducing noise, etc, which indirectly enhances detail of what you're trying to capture.
If you think M42 at 1 second, ISO 3200, F5.6 came out "ok" wait until you see it at 1~2 minutes, ISO 200~400, F5.6. You'll fall over.
The benefit of the tracker is to increase exposure time without trails. This allows you to take out the ISO, and thus reduce a LOT of noise. It reduces post processing time quite a bit when you don't have as much dramatic noise to deal with. Combine that with some average stacking and you can generate ridiculously clean images of the night sky. Plus, long exposure just lets in a lot more light, and you're usually limited to exposure times based on focal length--but with a tracker, you can just expose for as long as you want based on your tracker's accuracy.
I look at it in stops of light:
Aperture is static, wide open, widest you can use, so that you get as much light no matter what else you're doing.
ISO, each stop is a stop of light capture, but at the cost of noise and loss of dynamic range. Higher ISO gets you more information, but at the cost of noise.
You can reduce ISO, but each stop you lose, is a stop of light lost.
Shutter time, look at it in stops of light as well. 30 seconds... 1 minute... 2 minutes... 4 minutes... 8 minutes... etc.
You get WAY WAY WAY more light from that 4 minute shutter time, than you will from ISO 25600 on a 30 second image. And if tracking properly and accurate, it will be really really clean.