PCs are not common people. As such, you can be as rare as you'd like. That said, from my experience as a player, it's one of the most popular bloodlines (From an OOC standpoint). As for In Character, I don't think there are any numbers jotted down, if you want an estimate, ask the GM.
If this blog post is anywhere near canon, then they are more common than one might think.
The basic rules for bloodlines assume that Arcane bloodline is the most common (and it's the default for NPC sorcerers). I'd imagine that all the other core options (including Draconic) are about as common as each other, but less common than the Arcane bloodline.
If I recall correctly, the Inner Sea World Guide (and maybe its baby brother the Inner Sea Primer) has sections detailing all the core classes in the context of demographics on Golarion.
I certainly got the feel that everyone has probably run into a sorcerer in their life. A lot of people probably have one as a cousin or something. Most of them don't go adventuring nor do they attain many class levels.
Ethnicity and location also play a role in likelihood. I know Varisians are much more likely than others to have sorcerous blood. If you read Occult Realms (or is it Adventures?), Leylines are also responsible for a lot of non-normal humans (psychics, demi-human races, sorcerous bloodlines, etc.)
I flagged this, as I think this belongs in the Campaign Setting General Discussion forum, not Rules Questions.