We have a lot of wheels to get out today, so I'm going to publish this without some charts and graphs that I'll add a little later. The words will convery the point adequately enough for the time being, but charts and graphs always make a nice impression on management - those guy are short attention span theater, what can I say?
Anyhow, spoke strength is generally expressed Newtons/mm2. To get the functional strength of a spoke, you multiply that number and the spoke's cross sectional area. A CX Ray has a strength of 1600 N/mm2, and an approximate cross sectional area of 1.98mm2, for a total strength of 3168. I say approximate because I give the cross sectional area as if a CX Ray was a rectangle, but it's actually an oval so my stated cross sectional area is a bit high. The total strength and weight per spoke of an array of common spokes is as follows:
CX Sprint: 3564/5.2g*
CX Ray: 3168/4.25g
*Sapim hasn't published the strength yet, so this is a very conservative but very educated guess
Given these numbers, you can see that if you build a wheel using exclusively CX Sprint or Race spokes, you'd need fewer of them to have the same overall spoke strength. The dynamics of a wheel, however, allow us to engineer a little more elegantly than that.
The most loaded spokes by far are the rear drive side. They are loaded to the highest tension (around 20% higher than a front spoke, which is next), and are subject to the most dynamic forces. Without adequate total spoke strength (spoke per strength times number of spokes), a rear's drive side spokes will slightly elongate over time, and won't have adequate strength to cope with the dynamic forces that they see. The front spokes are fairly highly tensioned, but a front wheel is symmetrical and doesn't see near the dynamic loads that a rear wheel sees. A front wheel built with 20 Lasers, depending on the rim and hubs used, will do pretty extraordinary duty without batting an eye. The rear non-drive spokes? They're kind of along for the ride. They get loaded to a maximum of 50% of the drive side tensions, they see less torque than the drive side spokes, and the rear hub's left flange placement means that they don't have very hard work to do to keep things in line.
Given that, you want a ton of total strength on the drive side rear, and you need somewhat less on a road front (disc fronts act similarly to rim brake rears), and you don't need very much on the non-drive of the rear (or non-disc side of a disc front). Against that, you are going to balance weight. The lowest weight at which you can get adequate total strength is the best.
You also have a couple more things to consider. One is points of control. When you build a wheel, the more spokes you have, the more opportunities you have to correct minute rim issues. If the rim has a hard spot, more spokes allow you to taper that correction by making small corrections to several spokes rather than larger corrections to fewer spokes. This makes a better build, and the points of control also work for you in counteracting stresses that the road puts into the wheel. Many hands make light work.
Another thing is simply redundancy. My dad was an aeronautical engineer for his entire career, and did some pretty neat stuff. A trip to Udvar-Hazy with him makes for a very interesting day (and a lot of "I don't know if that's actually been declassified yet"s). Anyway, one of his cliches is that you strive for infallibility but you always use redundancy. If a spoke breaks on a 20 hole wheel, or a 24h rear, your ride is almost certainly toast - this is not a blanket statement but it's generally accurate. The more spokes you have, the less true that becomes. With 32 spokes, if one breaks you rip out the debris and ride on, maybe you have to open your brakes a little. On a race wheel optimized for lightest weight and absolute best aerodynamics, sure fine. On a wheel that you rely on all the time for all of your riding, or on which you are going to go out into remote places beyond "honey can you come pick me up?" Not so much.
Okay, so that was long, and miles to go before I sleep and all of that, but with that done you have a very good picture of a lot of what we're thinking about in terms of spokes when we recommend a particular setup.
Have a great Memorial Day weekend, and please keep in mind the staggering volume of day drinking that America does this weekend - please be careful on the roads.