Why do wind turbines have three blades these days although I used to see two or a large number (for low speed turbines )?

The reason why you want to avoid more blades than necessary, is that the addition of each additional blade has a diminishing return. And it doesn't become worthwhile to add the extra material, for the slight amount of additional power that it might capture. Also, each blade disturbs the air, and creates turbulence. It is the intent to minimize turbulence, and thus minimize the number of blades.

The reason there cannot just be one blade, is that it is inherently imbalanced. You need a second blade so that the rotor is balanced, and doesn't come to a standstill if it stops in the wrong position.

The reason why two blades are not used, is that the blade structures are cyclically loaded in compression and tension, due to the rotational pattern. When three blades are installed, the cyclic loading on the shaft of the turbine rotor is balanced, and doesn't become a problem for dynamic loading against activity of the yaw mechanism that adapts for wind direction.

In the old days, the blades were very heavy. Now that composites and CF are used extensively, three blades together weigh much less than a single blade from 20-30 years ago.

And everyone knows that three blades will produce more power than two in the same wind conditions.

I'm sure some design guy found that three is actually better balanced than two. There's no point in having four blades as they can only spin so fast anyhow. There are speed limiters on them to prevent disasters such as this

https://www.youtube.com/watch?v=7nSB1SdVHqQ

Of course, that one failed.

PS: Those blades are 130 feet long.

Three blades just have more power to capture energy from the wind than two. Three blades don't need to be as long, and the limiting factor today seems to be how long they can make blades and still transport them to the site.

In fact, I'm surprised we aren't seeing four-blade wind turbines these days. Maybe there's a balance issue or something.