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On the kind of bikes that generally have dynamos fitted to them, low spoke count wheels are usually not the most sensible choice. If you already have a rim to use then by all means. If you're using it on nice roads and your light, sure. Personally, being a fatty and racing stupid distances over stupid terrains I will stick with 32h when possible. I do have lower spoke count wheels but I do like not having to worry about any durability issues... at least until I crash them :)
What depth rim? 24H is quite standard on the rear for 50mm