The only way to really know is to look at sales figures from a major manufacturer like Schwalbe or Kenda -- who, as far as I know, aren't releasing numbers.

Plus, there are two 20" standards -- 406 and 451. 406 is common, but most tires will be for kids' bikes and BMX; 451 is much harder to find.

I don't think it matters in the slightest though. What really matters is the availability of good tires for your specific purposes. If you like the 406/451 tires you're using, and you keep a few spares, you're golden.