'06 LeMond Versailles, '04 S&M Beringer, '03 Quamen Bowls, '68 Raleigh Grand Prix (converted to fixed gear)
3x for 36h, 4x for 48h.
Crossing more spokes doesn't make your wheel stiffer, it just makes the angle at which the spoke pulls on the hub flange closer to tangent. Forces applied in that direction are less likely to rip your hub apart, but the wheel becomes heavier and slightly *less* stiff, which means you're more likely to bend rims. A radially spoked wheel is the stiffest and easiest to keept true, but your hubs will explode. So, there is a sweet spot, a point of balance between hub and rim longevity, and this hinges on how the hubs are *designed* to be pulled on.
The more crosses you have the flexier the wheel is and the freater the impact it can absorb, similar to butted spokes. If your 36h is big enough then it's probably worth going 4X, but lots of 36's aren't.