If you just want the card, you don't have to notify them. If you want the reward, you have to notify them in advance.
Here are the rules, terms, and conditions:
https://help.carnival.com/app/answers/detail/a_id/1173/~/guest-recognition---milestone-rewards-program
Just make sure your Carnival account accurately reflects your cruise count.
It would be extremely unusual for you to not automatically receive the milestone card.