To quote Sheldon Brown:
Traditionally, most bicycles have had 36 spokes in each wheel. British
bicycles, for years, used to use 40 spokes in the rear, and 32 in the
front. This was a better system for the consumer, because the strength
of the wheels was in better proportion to the stresses on them. It
makes things easier for the manufacturers, however, to use the same
number of spokes front and rear. This results in a front wheel that is
needlessly heavy, and/or a rear wheel that is not as strong as it
In the last decade of the 20th century, 32-spoke wheels became
increasingly common. Manufacturers tout this as an advantage, because
it saves a very small amount of weight (they don't mention that it is
also cheaper!) For most cyclists, the reduced strength and
reparability of 32 spoke rear wheels is a greater detriment than the
very tiny improvement in performance they offer.
In recent years, the same scam has been extended, as the industry tries to see how few spokes they can get away with before the
reliability gets so bad that consumers revolt!
These days it is common to see mass-produced bikes and aftermarket wheelsets with 24 or fewer spokes in each wheel. These are represented
as "premium" wheels, though they generally have off-brand hubs that
are a lot cheaper than genuine Shimano or Campagnolo hubs.
Naive consumers often fall for this scam, thinking the wheels must offer higher performance due to having fewer spokes. They don't
realize that these wheels make up for the lost strength of the missing
spokes by using substantially heavier rims! These trendy wheels look
lighter than traditional wheels, but they aren't. Some of these wheels
are unreliable -- and dangerous because if one spoke breaks, there are
too few others to keep the rim stable.
Many of these wheels also have nonstandard spokes that can be hard to find when a replacement is needed.