I've heard the 180,000 figure banded around for ages, as well as 60,000, even 20,000.
Personally, I don't seriously believe there are 180,000, I think thats way too high. I'd say its more like 60,000.
But the problem is, there is no easy way to find out. You don't have to 'register' to be a window cleaner, and as we know membership of the trade associations is no indicator. Lots of window cleaners will not admit to working if they are asked.
Also, do you class someone who does gardening with a bit of window cleaning as well? What about those that only do 2 days of window cleaning a week?
The definition of a "window cleaner" is not as easy as you might think, and its even harder to estimate how many people do it.
So the answer is, not 'I don't know' but rather 'you can't know'
-Philip