Click to see data.
My guess is that Derek Jeter has been seen in person in MLB games by more baseball fans (some multiple times) than any player ever. Maybe more than any athlete.
A pretty good guess is the number of fans who saw Jeter in home games: 52,272,638. Jeter played in 93% of Yankee games from 1996 through 2011, ignoring 15 games that Jeter played in 1995. The Yankees home attendance was 56,153,518. 93% of that is about 52 million.
I don't know how many watched the Yankees on the road but I figured that two million per season was a conservative estimate. That's another 32 million.
Jeter played 151 playoff games. Let's estimate an average attendance of 50,000 for another 7,550,000.
Jeter's total is about 92 million fans who watched him in person, not counting exhibitions.
If anyone has actual data for Yankee road attendance, please supply and I'll make that estimate more accurate.
Jeter could become the first 100 million fan player, a distinction that he would probably hold forever.