There is no set rule for this as they can be installed anywhere in your building. Generally though two sets is considered normal and will be placed at opposite ends of your building. One high up near the roof then the opposite lower to allow air flow.
The need for any vents at all will also depend on the amount of use it receives.
Please see log cabin air vents for more information.
Answers to most questions can also be found by typing into the search bar in the widget or our info center or please contact us to help you further with any questions you have.