How many paid developers work on developing Ubuntu? How many of them are on Ask Ubuntu?
When I say "work" I mean they get a monetary reward (salary) for working/participating in the development of it.
According to their own description, Canonical has over 400 staff and is focussing mainly on Ubuntu. 63% of the Linkedin-using staff is involved in development, so my guess would be something like 250.
Source for 400 figure: http://www.linkedin.com/company/canonical-ltd.