The more blades you have the more likely you are to get 'cascade effect' at very high revs. This happens when the speed of the blades causes the fan to become, in effect, a solid disc (in fact, this effect is also known as 'disc-ing').
The plastic 11-blade fan seems good up to about 5000 rpm (engine speed and with the small water-pump/fan pulley).
For sustained high revs the 6-blade, or 'export' fan is excellent, as is the old competition 4-blade. Less blades seem to increase the engine noise.
Who can remember the early 16 blade steel fans which 'disced' at about 4500 rpm? Useless for any high-performance use.
Cascading is also caused by the engine being in the way, thus restricting air flow to the fan