How many? More than 30 million.
Last numbers quoted was that out of 77 mil X360s sold, 46 million were XBL subscribers.
Yes, that number include the free Silver membership.
Let's assume 60-65% pay for Gold. That is only around 30 million Gold members.
No, that doesn't mean that over 30 million people don't use Gold. It's not as simple as taking the difference of consoles sold and Gold members.
You are assuming there is only ONE Xbox purchased per Gold member. Meaning that any Gold member who ever bought an Xbox only bought ONE xbox. My house personally accounts for 2 Gold members, but we have purchased a total of 6 machines over the 360's life cycle. Now, not sure how the silver plays in, but I doubt it's that cut and dry as others have stated.
On the flip side though, some households have 1 or 2 Xbox(s), but 4 Gold accounts (family membership).
Either way you look at it, there's really no way of telling what the actual percentage or number is, so people need to stop pulling random numbers, such as 30 million in your post, out of thin air.