From "The Bicycle Wheel" by Jobst Brandt, page 68:
The maximum number of crossing is produced when the spokes lie most nearly tangent to the flange. This number can be determined by dividing the number of spokes by nine. For example,in a 32-spoke wheel the maximum number is 3. If this number is exceeded, the effective flange diameter will be reduced, and spokes will overlap the heads of others causing gridlock.
If the flange diameter is unusually large or small, the number of spoke crossing must be reduced to avoid interference between spokes at the flanges. When the flange diameter is greater than twice the spoke spacing at the rim, such as in a hub with an internal brake, the "divide by nine" rule no longer works because spokes will reach beyond the tandent point on the flange and interfere with adjacent spokes. Spokes that interfere with adjacent spoke heads are difficult to replace, and they receive an additional bend that increases stress.
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I've always built 3x; when I did it with 28 holes, I found I had to order unusually long spokes and, if I had it to do again, I'd go for 2x with 28 holes. 3x with 32 or 36 holes seems about right although 4x, with low flange hubs, should work fine. With high flange hubs and 36 holes, 3x seems to be the most one should do.
-S-