I think the forum probably holds a much smaller number--perhaps 1/10th? But that's just for here--some people (like me) are "one-site only" types. I'd imagine there's quite a few out there who customize but don't make it publicly known (stigma over how others would see their doing this as a hobby ("He plays with toys!") or simply a lack of faith in their own abilities, compared to others.)
It's always fun when someone pops up who says, "Wow, I didn't think anyone else was doing this!" because it SO reminds me of myself when I first got into this (around 1999-2000)--I was shocked to find out others did, via the old Bivouac. Of course seeing the cool stuff there kind of made me doubt my own abilities so for years I was part of that "silent" group.
I don't think you'll ever get close to exact numbers but if I had to guess, I'd number it easily into the thousands, covering all 1:18 scale lines and possibilities and from what I see (with the arrival of newbies getting into the hobby), it's still growing.