Aart, look at the screen name. Probably someone from Arkansas. Of course they could be out-of-state.
Tractors are different than normal coops because you are always moving them. That keeps thing interesting and stirred up. They are not as likely to be bored. Regular rules of thumb don’t apply because your management techniques are different than what that rule of thumb is based on.
It’s not how much space is in the coop by itself. It’s how much total space is available to them when they need it. An 8x12 is big for a tractor. I don’t know if the 4x6 coop is included in that or extra. Usually a tractor includes both the coop and run as a protected area so you should have the total space available.
You will be fine with 9 chickens in there though that may be getting close to the limit if you don’t move it much. I don’t know how often you’ll need to move it with nine. Maybe not a whole lot. Usually they are more crowded and you have to move them pretty often because the poop builds up and starts to stink.
How many could you possibly shoehorn in there? That’s real hard to answer. Part of that will depend on the personalities of your chickens. Each chicken has its own personality and each flock has its own dynamics. Part of that will depend on how often you move it. I’m not even going to try to give you a number, there are too many unknowns.
X 2 - tractors are a whole other animal.