How many people have to die before Prince William becomes king?

Answer 1

That is a rather macabre way of asking the question, but Prince William is second in the line of succession for the throne of the British Commonwealth, after his father, Prince Charles.

So when the current monarch, Queen Elizabeth II, passes away: his father would then inherit the throne (presuming that he too is still alive, and chooses not to abdicate in favor of his son) followed by William himself.