I would use 36 spokes for that rim or any other similar shallow cross-section rim. Shallow rimmed wheels are traditionally built with 36 spokes for a good reason...durability.

Deeper rims not only produce laterally stiffer wheels, they are vertically stiffer and distribute your weight through a larger portion of the rim, thus reducing spoke fatigue. This is why shallow cross-section rim wheels tend to be built with high spokes counts (to reduce fatigue and prevent spoke breakage) and why newer deeper rim wheels can have dramatically fewer spokes and run higher spoke tension.

I don't know how rough your ride, but you can probably get away with 32 spokes, but you run a greater risk of premature spoke breakage. If you were to use a deeper rim, such as a Deep V or Kinlin 30mm, you could go as low as (24-28) spokes and still have a very stiff durable wheel.


Edit: while we are on the subject of durability, use double butted spokes if at all possible. DB spokes are not only lighter, then are more fatigue resistant. Your wheel will be lighter and last longer...its a rare instance of having your cake and eating it too.