CyanogenMod is the most famous ROM software companies out there. They are famous for supporting almost every Android phone and bringing it up to date with the current version of Android. A couple days ago Steve Kondik posted on Google+ that they had just pasted one million unique official installs.
One million does not seem like a lot when there are over 700,000 Android devices activated daily, and over 200,000 million devices so far. In actuality, there are probably millions of people that use CM; not all builds for phones are official builds by the CyanogenMod team. Still, 1 person in every 200 use this ROM. Remembering that most people are not hackers or are afraid of doing stuff to their phones, this is a huge chunk of the market.