According to JAR-OPS
An aircraft with 2 or more engines flying over water away from land, 120 minutes in cruise speed or 400 nautical miles (witchever is less) requires life rafts and 2 emergency locator transmitters.
There should be enough rafts so if one would fail (with largest capacity) the other ones can accomodate all the passengers.
ex) if 50 passangers are travelling you need 3 rafts with the capacity of 25 persons.
For one engine aircrafts the reqirement is 30 minutes in cruise flight and 100NM
For ETOPS operation I am note sure if it is any different requirement.