A small electric fan is a motor, a set of blades and a guard, and almost every decision in it is a trade with the others. The number of blades looks like a styling choice. It is closer to an accounting result: how much air can be moved by a motor of a given size, at a speed quiet enough for a desk, from a part that can be moulded cheaply.
Fewer blades, faster rotation
For the same motor, fewer blades can be spun faster. More blades add surface area and drag, which asks for more torque and usually means running more slowly to stay within the motor's budget. A two-blade fan moves a great deal of air but needs careful balancing and tends to be noisier; a five-blade fan can be quieter at low speed but needs a larger or more capable motor and a bigger moulding.
Three sits in the middle. It is the smallest number that balances naturally on a hub — a two-blade rotor has a strong tendency to wobble because an imbalance has nowhere to be averaged out — while keeping the mould simple and the torque modest. Around Shunde's small-appliance belt this combination is easy to tool, easy to balance and easy to pack.
The twist matters more than the count
A fan blade is not a flat paddle. It is twisted, with a steeper angle near the hub and a shallower one at the tip, because the tip travels much faster through the air than the root does. If the angle were constant, the outer part would do almost all the work and the inner part would mostly stir. The twist is an attempt to make each radius contribute a similar share of the pressure rise.
The mould has a say here too. A blade with a smooth twist and a rounded leading edge releases from a two-part tool more reliably than a sharply undercut one. Shape that cannot be demoulded cheaply is shape that will not be made, however good it looks in a simulation.
The guard is part of the airflow
The wire cage in front of the blades is specified by safety rules, but it is also an obstruction. Every wire leaves a wake, and the spacing between wires changes the noise: a regular grid can produce a tone at the frequency at which blades pass the wires. Designers vary the spacing or the shape of the guard to spread that energy rather than concentrate it. A fan that hums at one note is often a guard problem, not a motor problem.
Oscillation adds another constraint. The whole head swings on a gearbox that has to be cheap, quiet and able to reverse direction for years. A heavier rotor makes every reversal harder. Blade count and blade weight therefore matter twice — once for the air, once for that small gearbox.
How to judge one
Do not count the blades first. Watch how it starts and stops: a well-balanced rotor coasts to a halt without the head shivering. Listen at low speed, where guard noise is most obvious. Then look at the twist from the side. The number of blades is the last thing that was decided, and the least interesting.