Three, obviously
GILES: "And you are called?"
KENDRA: "I am the Vampire Slayer."
**************************************************
GILES: "Um, th-the new Slayer is only called after the previous Slayer has died. Uh... Oh, good Lord! You were dead, Buffy."
------------------------
And don't forget that Xander's first words on seeing Faith were something like, "I'm guessing there's a new Slayer in town."
-------------------------
It's there in black & white.
Think about it. Buffy was not an athelete, she did not have her strength or agility before she was "called" and trained. She says herself that she became the Slayer at the age of 15. Therefore, Kendra also became a Slayer when Buffy was drowned by the Master. Slayer is a birth-right, but you don't become one until it's your time - until you're called. And a new one is called each time one dies.
The old chestnut that "there's only one per generation" is obviously wrong, since some Slayers don't live a generation, which is defined as 30 years. It's poetic short-hand for there's only one at a time. But Buffy changed that by surviving her death. Buffy has left her permanent imprint on the Slayer legacy.
I agree that has to be several women lined up to assume the title at any time. But many are not called. There are at least two Slayers now "active". Since Buffy has died again, I know think that there should be three. If she keeps dying there may be a population explosion of Slayers.
If you don't believe that Faith is a Vampire Slayer, then you tell her, but make sure that your insurance is paid up.