The number of bikes chained to the wire fence at our local primary school (population 340 kids) varies enormously over the seasons. On the warmer, settled days there will be anything between 20 and 40 bikes. On cold, windy days there will be only a few, maybe none.
So what is the capacity we should aim for? The presence of a bike shelter may stimulate more bike usage, so perhaps (in our case) we should aim for a 40-bike storage facility. It may also be worth researching the cycling numbers in schools that have introduced the measures that would encourage more cycling - but paying attention to how these numbers hold up over time. As a rule of thumb I would have thought that 10% of pupils cycling most of the time is a bare minimum, and the target should be more like 30-40%. In this case the capacity for our shelter would be over 100! The standard retail cost of that would be unattainable, so how can schools possibly manage it?