ELI5 Why don't wind turbines have six or eight blades instead of three to catch twice as much wind energy?

Wind turbines wear out. Replacing 3 is easier than replacing more.

i also heard that half the problems they have are the turbines spinning TOO FAST.

they really dont want to "catch twice as much energy"

Three blades is the most efficient in terms of power generation, cost and stability. More blades would generate a bit more power but also cost a lot more to make and maintain, it simply isn't really worth it from a dollars perspective. You'd be better off spending that money on another turbine.

Because the turbine works by catching the wind passing through the area of the blades, but the more blades you put on the less wind is able to pass through.

With lots of math and experimentation its been found that turbines with more than 3 blades actually capture less energy because they reduce the amount of wind that passes through.

You can see this if you imagine what would happen if you just kept adding blades - at a certain point its just a solid disk and no air passes through at all, resulting in the turbine not spinning. You'll need to remove blades from the turbine to let air through to turn it and it will be when you have three blades that you find its turning fastest.

This video explains this and much more very succinctly. ▶Watch on YouTube

One thing with wind energy is that you can only absorb so much kinetic energy from wind before it stops being wind.

The blades are big enough, and can adjust their angle, such that they already make plenty of power in any realistic amount of ambient wind. If they caught more wind, it would be more stress on the structure and have to be built in a more expensive way, or worse, completely redesigned.

The power of a fan scales with the swept area (blade length), more than the number of blades.

You get hugely diminishing returns with more blades, because each blade will encounter more turbulence caused by the blade ahead, as opposed to the power-creating flow that you want. It wouldn't be worth the significant extra cost of building and transporting them, especially once you add in that the tower needs to be much stronger to support all that weight.

There's a balance in play with these.

The energy comes from the spinning in the centre of the fan blades. Not enough spin and not enough energy, too much wind required and not enough energy generated over time.

More blades doesn't always equal the ideal spin rate. They have a retard mechanism in place already to try to prevent them from spinning too fast. More blades would just stress that mechanic even more.

The next great improvement in that design will be the ability to capture more at the centrepoint and reduce any potential waste in the system.

Maybe then they'll build them with more blades.

I'm guessing that there is likely some constraint imposed by the maximum capacity of the bearings used. Those blades are insanely large, and weigh extraordinary amounts. The bearing supporting three blades probably isn't technically maxed out, but after you factor the wiggle room needed for safety, it might be difficult to engineer a bearing to handle the extra weight of more blades.

Turbines can't turn faster than the wind is blowing, so there are diminishing returns from adding more/"catching more wind". Three provides nice stability (which two definitely lacks); adding more blades costs more and can lead to turbulent interference.