Extract from the sheldon brown site.
How Many Spokes?
Up until the early 1980s, virtually all adult bikes had 72 spokes.
32 front/40 rear was the standard for British bikes, 36 front and rear for other countries. The exception was super-fancy special purpose racing wheels, which might have 32 spokes front and rear.
The Great Spoke Scam: In the early '80s a clever marketeer hit upon the idea of using only 32 spokes in wheels for production bikes. Because of the association of 32 spoke wheels with exotic, high-performance bikes, the manufacturers were able to cut corners and save money while presenting it as an "upgrade!" The resulting wheels were noticeably weaker than comparable 36 spoke wheels, but held up well enough for most customers.
Since then, this practice has been carried to an extreme, with 28-, 24-, even 16-spoke wheels being offered, and presented as it they were somehow an "upgrade."
Actually, such wheels normally are not an upgrade in practice. When the spokes are farther apart on the rim, it is necessary to use a heavier rim to compensate, so there isn't usually even a weight benefit from these newer wheels!
This type of wheel requires unusually high spoke tension, since the load is carried by fewer spokes. If a spoke does break, the wheel generally becomes instantly unridable. The hub may break too; see John Allen's article.
If you want highest performance, it is generally best to have more spokes in the rear wheel than the front. For instance, 28/36 is better than 32/32. People very rarely have trouble with front wheels:
Front wheels are symmetrically dished.
Front wheels carry less weight.
Front wheels don't have to deal with torsional loads (unless there's a hub brake).
If you have the same number of spokes front and rear, either the front wheel is heavier than it needs to be, or the rear wheel is weaker than it should be.
