You really need to look back to the 2000 primaries, since it wasn't really a contest in 2004. Here are some stats I found from 2000:
http://www.thegreenpapers.com/PCC/Tabul.html
They included a summary of the primary and caucus popular vote:
Looks like just under 20 million people voted in the Republican primary. Assuming 200 million Americans of voting age, and 100 million in each party (splitting independents), that's around 20% turnout.
Assuming 20% turnout this time, and guessing that 30% of the vote will be enough to win, given that there are 6 serious candidates instead of 2, Ron Paul would need 6 million votes, at least.
How many supporters does he have? If he's polling at 5% nationally, that would be around ten million. If his actual support is higher than the polls indicate, we're that much better off. Now if we can just get all of them to vote in the Republican primaries....