If you are going to install multiple vents they should ideally be equidistant that is evenly spaced from the edges of the roof and each other.
Installing whirlybird roof vent.
This video demonstrates the installation of a whirlybird roof ventilator on a trimdek metal roof.
Intake vents in addition to the turbine on top you should have an intake vent on either end of your shed to let the air in.
This technique is also known as cross ventilation.
For example if the length of the roof is 21 meters and you want to install 2 whirlybirds you could measure 7 meters to the centre of the first whirlybird then 7 meters from there to the centre of the second whirlybird leaving a final 7 meters to the other end of the roof.
If you re a qualified tradesperson you may like to follow these for whirly bird vent installation also.
A roof vent also known as also known as turbine or whirlybird vents are installed in shipping container roofs.
As they spin these vents suck the air out of the container.