Your guest ID number will be pre-printed on the forms that you get when your documents come. The forms on line are a convenience, but it is actually easier to use the ones in the document pack as they are partially completed.
You need a cruise contract for each person cruising (there is a line for parent of minor to sign.) On the other forms, it is one per stateroom UNLESS you are using multiple credit cards. Each KTTW can be tied to a different card if you desire--in that case, you need a charge authorization for each card being used.