…but what I would like to know if someone can provide a breakdown of how many in each category?

It’s maybe not quite as accurate as checking on how many people have crossed the vesting thresholds of 10, 40, and 70 MVests, but if we’re using Steem Power levels of
5,000-19,999
20,000-34,999
35,000-49,999
we can then look at steemitboard.com or steemfriends.org for their (slightly different) SP rankings and come up with some close estimates of the number of Dolphins in each subclass:

5,000-19,999:
1633 (steemitboard)
1627 (steemfriends)

20,000-34,999:
293 (steemitboard)
295 (steemfriends)

35,000-49,999:
104 (steemitboard)
107 (steemfriends)