how much of the population of seattle is sinless? just the barrens? half of the city? what's the global figures, either in your campaign or in the world?

One of the old SB's (I think it was 1st ed Seattle) actually gave a percentage. I think it was something like 15% but I'll have to go digging to find out for sure.

All of the books usually give an estimate because, of course, being SINless they are not registered so you can't count 'em.
In general, though, the SINless are confined to the local city's equivalent of the Barrens, which makes sense, since they lack basic rights like renting, owning, social services, etc. If they were in the city, by definition they would be renting something and have a job and all (even if it would be crummy) - random groups of squaters notwithstanding.

I think imperialus' estimate is about right though.