How many vent stacks should a house have?
Most homes will merge the vent pipes as they go up the walls so that you only have one vent exiting the roof. All vents need to slope upwards, so the guest house couldn't run the vent underground to use the vent stack in your home.

That said, if you notice that drain lines are gurgling after the water goes down (indicating suction) or are slow to drain but have no clogs (indicating it's trying to overcome the air pressure in the lines) then there may be a blockage in the vent or it may not be properly installed.

Note that for some situations, you can also have an air admittance valve to provide a one way vent (fresh air can get sucked in, but sewer gases cannot escape), which allows you to vent plumbing in a kitchen island or other location that cannot go up and out the roof.

For the odd smell, it's difficult to diagnose over the internet, but it could be a dry trap, a failed wax seal under the toilet, or a dead animal in an exhaust vent or inside a wall.