That's so funny. Where did they ever come up with that number? I can see Osama and his crew sitting around a campfire, riffles in hand, joking about what the perfect number of wives would be.
Its not a specific number; they believe they get x amount of virgins depending on how many people they kill. If they kill 30 people, they get 30 virgins. If they kill 72, they get 72. It just depends on how many they kill. Nice people.