I was at university in '83 and I don't remember there being more than a half dozen people there that were seriously hooked on windsurfing. I doubt things have changed much these days.
The big difference was the number of people who had tried the sport or had windsurfers stashed at the families shack, but really windsurfing was just another cool toy or a diversion on a summers day. These days folks probably have a jet ski or mini motorbikes or some other toy though I still see quite a few older longboards at the beach in the hands of the idle yoof (some board short wearing and some bikini clad).
And there is the difficulty, do you limit the count just to those with gear less than five years old (surely anyone with older gear isn't serious about the sport

) , do you count those that have windsurfed once or twice or haven't sailed for a few years but wont sell their gear because........well the question seems a little pointless.
And my forum quote is......
"An intelligent hell would be better than a stupid paradise"