Fewer spokes are lighter (by about 4.5-5.5g/spoke depending on the spokes used) and more aero by a small amount.

Rim stiffness, number and diameter of spokes and hub geometry all play into a wheels lateral stiffness. If you use stiffer rims you can get by with fewer spokes and still have enough lateral stiffness. But stiffer rims are usually heavier as well, because rim stiffness is largely determined by the rim's cross section. So that's a tradeoff too.

What an individual can get by with depends on their riding condition, weight, riding style. Rider weight is not the only factor.