Generally more spokes is stronger than thicker spokes. But all the coaster brake hubs i've had with more than 36 spokes are low grade hubs. I would not recommend them for adults.
Stainless steel spokes are far stronger. DT straight 2.0mm stainless are the best. I used them on my bicycle built for 4. 48h rims and hubs. 700 pounds of weight. Never broke a spoke in 5000 miles. Okay I did break several rims. Black spoke are the weakest. You will be breaking them quite often. Brass nipples are stronger than steel or aluminum.
Ive seen spokes in many thicknesses. TO keep us confused, the bicycle industry has used 3 different measuring methods. SWG, mm or decimal inches. 14g is about the same as .080" or 2mm. To really mess with us, a large chicago based bike company would list all their spokes as .080" or .105" or .120" regardless of who made them or what units those companies manufactured them in. Anyhow 14g, 13g, 12g, 11g, 10g are all available. if you look hard enough.
Over size spokes can require over size drillings in the hubs and rims. The Schwinn Heavy Duty/Twinn wheels used specially drilled rims and hubs. Drilling out old school coaster hubs is a pita. Most newer coaster brake hub flanges are just sheet metal.
Trail riding with a coaster? When I was a teen I was riding dirt trails on my Schwinn Typhoon with the bendix 2 speed automatic. I got left in the dust all the time. A single freewheel and hand brakes is going to be a lot faster and you can get 40 or 48 spoke hubs and rims with a single speed freewheel. Lighter stronger faster.
