If multiple Vampires you control (possibly including Kalastria Highborn itself) are put into their owners' graveyards at the same time, Kalastria Highborn's ability triggers that many times.

If the targeted player is an illegal target by the time the ability resolves (for example, if that player has left a multiplayer game), the entire ability doesn't resolve. You can't pay and you don't gain life.

You target a player when the ability triggers. You choose whether to pay as the ability resolves. You may pay only once per resolution.