I have built all my own wheels for the last 8 years or so so i know what goes into it and how it works. I have read many things (most things actually) about wheel building. I'm not a guru or anything like some pro builder but I feel confident in my knowledge.
as to whether a "factory machine built in 10 seconds" wheel is better if its 18/24 or 32 spokes i'll let go unsaid, because no one knows to what quality they build the wheels, but from what I have encountered they are almost all crap workmanship needing truing very quickly (the sign of mediocracy). also most of the 32 spoke wheels i have seen are straight guage spoked, non eyeleted cheap rimmed (so they are crap to even begin with). since butted spokes are more flexible and therefore last longer.
but if you are saying a handbuilt (and correctly built) 20/24 will be stronger than a 32 all else being equal. its just doesn't work that way.
more material means stronger.
I have never had to true any of my own wheels, surprise surprise. and none of them have ever lost tension or had individual spokes go out of tension (measured with my park tensiometer). I have done some serious drops to flat on all my wheels (like a meter sometimes). and do it every day. I dont baby my stuff. and I'm not very "smooth" either. I have no problem hitting a sidewalk edge at 30km/h with my rear wheel only partially unloaded.
In fact, high numbers of spokes are actually responsible for spoke breakages, in a way - the more spokes, the lower tension they're at. How do you prevent spoke breakages? High tension, because fatigue is what gets them. Wheels with lower spoke counts have no choice but to have high and even tension and therefore are automatically built to a higher standard. I've inspected dozens of pairs of Shimano's 16/20 wheelsets, and never found a loose spoke.
But hey, if you want to rag on hubs using cartridge bearings, I'm happy to jump on board.
i build my wheels to the max tension of the spokes where you can (unless the rims set the limit), and on the lower tension side it gets up to what it gets to.
the more spokes you have the more even tensioned the rim will be. the fewer the spokes the stronger/heavier the rim have to be around the spoke holes to prevent cracks. from what i have read the fewer spokes you have they heavier the rim has to be and thats the wors place to put weight. in the periphery.
regarding fatigue: all spokes gets unloaded everytime they hit the bottom when you roll since they are now in compression.
my own findings is actually that its the hubs flanges that is most often the limiting factor in spoke tension. some hubs like chris king and several others have an exposed bearing next to the cassette carrier, and if you put max tension on these spokes this bearing seat will expand and then the bearing is not seated properly and you ruin the hub.
on a sidenote i was discussing with a guy at work thats been into riding for 30 years or so about these new low spoke builds and he just told me; why take a chance?? why take a chance that the wheels will ever go out of true and/or break?? He didn't see the point. and to be honest neither do i. I rather build stuff that will last forever and dont worry than be on the edge of durability. its not that wheels are that expensive its just that it takes a lot of time to measure the hubs/rims then ordering stuff and building them up. its easily 10 hours for a set from zero.
I think the current low spoke count/weird designs is mostly fashion (like all campag wheel), and the only number the customer easily understands (weight) that dictates how todays road wheels are built.
all i can say is buy what you want. if you want a 24 out back wheighing 700g go ahead, just don't be surprised if its very flexy sideways and breakes within a year. remember this is what you wanted! and you go it.
and for the new first time builder: if your set comes in at a kilo or more. dont worry. they will last for a lifetime of abuse probably if you did it right.