To answer the question 'How many traps per hectare', it is simple to say:
You MUST organize your Pest monitoring based on the history of the insect pressure. In fact, the devices are uses as sensors and need actual insects to alert the grower when arrivals are been detected. Therefore, target high pressure areas !
Of course, implementation of a sensor network may help to work out models and anticipation...
This considerations must be associated with those of "Where to place my connected traps".