In the height of in-season madness I wrote this list of blog topics on a whiteboard on the fridge, and now that it's a bit less frantic I have a chance to get to some of them.
A customer asked this summer why there was no 20 spoke front wheel option for disc brakes, and followed that up by asking why there is a 24/24 option for disc but not for rim brakes. That the questions were asked by the guy who asked them was notable to me, because he's fairly switched on in terms of knowing about bikes, so I figured there might be a lot of people with the same question. So it made the whiteboard list.
There are a few reasons why there is no 20 hole option for disc brakes. The first is that a rim brake wheel transmits no torque from hub to rim. Transmitting torque from hub to rim is a fairly stressful activity for a wheel, so a wheel that does it needs more strength than a wheel that doesn't. In order to effectively transmit torque from hub to rim requires crossed spokes. The spokes in a radial laced wheel would just bend when you hit the brakes, and then eventually do a bad job of transmitting the brake torque to the rim. I still get a chuckle out of one company having introduced radial laced disc wheels, claiming they'd tested them and they worked great. We didn't even have to be the ones to call them out on it! But it's actually sort of improbable to do 2x lacing in a 20 spoke wheel as one spoke's head tends to interfere with the body of its neighbor spoke, and interlacing them as is the common convention just ain't happening without tortuously bending the spokes.
A rim brake front wheel is symmetrical, while a disc brake front wheel is dished. Symmetrical wheels are inherently stronger (it might be better to say "more stable" instead), all else being equal, so conversely a 20h disc front would be weaker than a 20h rim brake front. There is also the tension imbalance between the spokes on the disc and non-disc sides. With a 20h wheel, you have a fairly long span between spoke holes in the rim, and the longer the span, the greater the likelihood that you won't be able to get spoke tensions evenly matched throughout. That potentially takes away all of your margin for error against having a non-disc side spoke go slack, and then break. Plus the greater stress that goes on the disc-side spokes at all times, which then goes up whenever you hit the brakes. As aero disc-brake bikes become more of a thing (and they're already a thing, so they're sure to become more of a thing), we're sure to see wheel designs which "address" this. Because nothing says aero like a low spoke count. Actually, deep wheels say aero more than a low spoke count, and deep wheels with low spoke counts say aero the MOST! And with straight pull spokes, it's fairly easy to get around the crossed-spoke interference bit, albeit with some heavy compromises. But the bike industry is all about hiding compromises in order to bring out stuff that's differentiated and looks really cool! Do I sound jaded?
As to why 24/24 makes sense for disc and not road, as Sheldon Brown has said, if your front and rear wheels have the same number of spokes, either your front wheel is overbuilt or your rear wheel is underbuilt.
I've run long so I won't go into equivalencies between overall spoke count between disc and rim brake wheels now, but that will come later.