The sawmill has extensions, if you want more boards produced if the main sawmill is full. If you go into the global mod for that sawmill you will see on the left hand side a list of the extensions. Scroll down to the extension you want to use and scroll over to boards or resin and take out what you want. It will spawn in the spawn areas on the side of the sawmill
I do not believe you can spawn more than three but the flatbed autoloader will hold 16 so I just pull up next to the spawn point and keep hitting the respawn button on my controller until it is full. So much faster to have two 4 or 6 extra spawning points.
be sure to activate the extensions in the Global Company settings.
The sawmills with 2, 4, 6 has extra "boxes" These are for auto loading into the other forestry factories.
So to place the, for example, the pallet maker. When placing the pallet maker you will see a solid square. Place that square inside the sawmill outlined box. When you go to spawn the boards at that particular extension the boards will automatically go into that factory without having to manually place them there.
This only works for the factories that have the boards as inputs. Do not put the paper and chipboard factories in the squares because they take woodchips as inputs.