A good rule of thumb is one pair 1.5 metres from the end of the ridge line for each section of roof, and subsequent pairs evenly spaced approximately 4.5 metres apart, with the final pair approximately 1.5 metres from the other end. The average home may require between 2 and 4 pairs of these ventilation tiles.
Think of a pot in which you are steaming vegetables. To keep more steam in, you create a small opening with the lid. To allow more steam to escape, you allow a larger opening of the lid. So be aware, the ventilation of your roof space will be more effective by erring on installing more pairs rather than less.