Per plan. Not per device or person.
You just buy a plan, designate a user name/password and start using it.. you can share it with as many ppl you want, but obviously the more ppl use it the quicker it'll drain.
I usually lie to my DD13 that I got the cheapest plan & need it for work... otherwise she'll hound me for the password & use it to be online instead of having fun on activities which is the whole idea of being on a cruise.
I buy the $89 plan & it lasts me the entire cruise (7 day) even emailing attachments, using FB, IG and texting pics to friends/family.
It's awesome - highly recommended if you need to stay connected... DCL did improve massively in their connectivity in the past couple of years.