actually, modern wheels often have FEWER spokes than classic bike wheels, but this has nothing directly to do with threaded freewheel vs freehub+cassette...
36H was pretty standard in the 70s/80s, then they started down to 32, 28, 26, even 24 or less. I've seen 16 spoke wheels on modern race bikes.
fewer spokes makes a lighter wheel, but if anything goes wrong, you're waiting for a sag wagon. with 36H, you can break a spoke, and probably ride home just by setting the brakes a little wider.
you can get most model hubs in a variety of spoke counts. if you're going to build a new wheel using your existing rims, obviously, the hub has to have the same spoke count as the rim.. if you're going to build new wheels, you want hubs, rims and spokes all to have the same count. the number of spokes and the size of the hub flange, and the spoking pattern determines how long the spokes have to be.