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.

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

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 {B} and you don't gain life.