Well, some recent numbers were in this press release:

In 1983, Marriott launched the world's most successful hotel guest loyalty program. Today, Marriott Rewards leads the industry by a large margin with 20 million members, and is now offered in 65 countries outside the United States. After joining the program, Marriott Rewards members double and often triple their number of stays. Members also earn free stays 30 percent faster than with any other major hotel program, and Marriott Rewards offers more resorts, spas and golf than any of its competitors.

The full thread is at http://www.flyertalk.com/forum/showthread.php?t=323181

So, with 20 Million members in MR, I would think the number of Plats to be a very small number of that total. For debate sake, let's say .25%, which would make 50K Plats worldwide, though most of course would be in the USA. Anyone think those calculations even close?