Very few, but I have run into 2 (maybe 3) that had waiting lists. That it's not more common suggests there's a mismatch - it's entirely possible we simply have more supply than demand, in general. A nice situation for us as consumers, but not for us as instructors.
There's absolutely more supply than demand.
The whole thing before about "it's all karate to the public" - I may slightly disagree with that sentiment, but let's say it's true.
A Google Search for "martial arts {my town} yields over a dozen results, and it's not a big town. Expand 10 miles (which takes in 2 more smaller towns and a few large villages) and you get 30+.
Search for "tennis {my town}" and you get 3. Swimming, 2. Gym, 8.
As I'm putting MA in the leisure category, it shows there is a large supply with much choice.
Edit: I forgot the entire point of this post!
This shows something strange.
Tennis is on TV (the non subscription free to air channels) for every major tournament.
Swimming gets loads of TV coverage during the Olympics and the like.
Gym membership is advertised everywhere around here - local radio, billboards, flyers in the post.
To see MA on (free) TV you need to wait for the 5 minute highlights of the Olympics - or resort to YouTube.
The only advertising for MA I see is a couple of tiny adverts in the "active kids" mini magazine the school sends out, and we've had 2 flyers handed out by our kid's school (one of which I supplied to the school for 'my' club).
So, as good as zero advertising yet enough demand to keep that many clubs operating? Would extra advertising change that or is it really that MA is an activity that people actually look for if they have an interest?