Simply knowing the number of Platinum guests would be less helpful than knowing the typical number of Platinum guests on specific itineraries.
I would guess there are many people who reached Platinum while their kids were young(er) and then stopped cruising on Disney. These people wouldn't affect current policies, unless they start cruising again.
I would also guess that longer and unusual itineraries have a greater proportion of guests who are Platinum. People who have cruised often are likely to have enough vacation time for longer cruises, and Platinum guests are both able to take advantage of early booking and likely to want to sail on itineraries they haven't yet experienced.
On the other hand, I'd guess that weekday 4-night cruises tend to have lower proportions of Platinum guests. I'm not sure about weekend 3-night cruises -- those would appeal to the "take a lot of short cruises because I live nearby" Platinum people, but not the "I really love the ship and want to spend as much time on it as I can" people.
I am curious whether the Pearl rewards will be geared more toward people who typically take longer/unusual cruises or more toward people who take a lot of short cruises.