if your definintion of webmaster is any owner of a website, I know you can get your alexa rank down to about 7,000,000 So i would say 10Mil wemasters.
up possibly 80% only because their are new members registering everyday...which more likely are webmasters.
I think your guess is far less than what you think. There are much more, maybe few millions but nobody knows the exact number unless a survey is being carried out.