Weight limits for a wheel with a certain number of spokes depends mostly on the gauge of the spoke, also the material that the spoke is made out of, as well as the type of spoke (straight, butted, bladed, swaged, etc.). Also, the pullout strength of the holes on the flange are a consideration, but these are usually higher than the failure strength of the spoke (unless, of course, you do something inadviseable, such as radially lace a high-flange hub...) Because of all these different concerns, it is d@mn hard to make any generalisation based solely on spoke count.
As far as manufacturers specs are concerned, yes, they all have maximum limits determined by the builder. Maybe you should e-mail the manufacturers that you are interested in.
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