I have never seen a source that explicitly states a number or percentage of shadowrunners in the population. However, I think there are reasonable guesses that can be made.
First, the SR5 core rulebook states on p. 442
Shadowrunners by default are SINless...
There are, of course, exceptions that are intended to be rare (i.e. Shadowrunners with the "SINner" quality). So I think it's safe to assume that the Shadowrunner population is, for the most part, going to be a subset of the SINless population.
Second, according to System Identification Number entry on the Shadowrun Wiki:
By the 2070s, most countries participate in the in the Global SIN Registry (GSR). To do so, more than 90 percent of their recognized citizens must be issued SINs...
I'm not 100% sure where they get this information from, but the page references both the SR3 and SR4 book. I don't have the SR3 book, and it wasn't in the SR4 book, so I'm assuming it's mentioned in the SR3 core rulebook.
This narrows down the search to, at most, 10% of the world's population. However, it's easy to assume that the Shadowrunner population is significantly less than this. A few points are:
- 90% is the minimum requirement to participate in the GSR. Odds are that most nations will exceed that number.
- It's a reasonable guess that not EVERY SINGLE SINless person is going to turn into a criminal-for-hire.
Unfortunately, to narrow down the number any further would require us to do a lot of speculation, and (as you have suggested in your sub-question) this number will also be affected by how you define "Shadowrunner". It is, at least, reasonable to say that 10% is a good starting point from which to work.