Making educated guesses is all very well but it makes much more sense to just ask an expert so I pinged the team at Feedburner (Google) to find out some real numbers.
The figures they sent back are for Q2 (they are still compiling the Q3 figures) but we know from the Feedburner home page that they have burned 1,023,739 feeds (as of 14 Oct 07).
Here is what Feeburner had to say about the data:
Feed subscriptions are not unique. Individuals frequently subscribe to more than one feed. The 65 million number at the end of Q2 represents the number of subscriptions that FeedBurner delivers on a daily basis to more than 190 countries. Today, that number is closer to 75 million.
Even if Feedburner has a 50% market share of feeds this means over 150 million RSS feeds are being delivered every day.