Watching S3's "Consequences" last night with Shempy, I found evidence that supports my theory of TWO Slayer lines past PG, and thus three Slayer lines past The Gift:
(usual disclaimer: retranslated from the German dub, so the exact wording might be a bit off)
Mayor Wilkins, to Angel posing as Angelus: "Take your time in killing her (Buffy). We wouldn't want a new Slayer to appear around here any time soon, not all of them could turn out as swell as my Faith."
While the defenders of Word-of-Joss ( 😉 @ Shempy) can argue that the line is not hard and fast proof - given that Wilkins doesn't necessarily know the in-depths of how Slayers are called - I do consider him to be pretty well informed, being a major power figure both politically and supernaturally. And besides: I have now given you canon evidence in favor of the "multiple lines" theory, while to my knowledge absolutely no canon evidence exists for the "single line through Faith" theory! (Remember that Word Of Joss is not the same as canon. As long as it's not on screen or in an official comic panel, it's just Joss' personal opinion - certainly not to be discarded lightly, but not canon.)