The Speedway has never released official attendance figures and has never stated publicly how many seats there actually are.
This article from USA Today dated May of 2004 says
INDIANAPOLIS — Indiana National Bank asked Tony Hulman to share Indianapolis 500 attendance figures when he borrowed an estimated $700,000 to purchase Indianapolis Motor Speedway in 1945.
The bank never got the figures. Hulman paid off the loan.
Although Hulman died in 1977, his secret has lived on. No one connected with the Speedway has confirmed the size of any race-day crowd, and the limited number of people who know show no indication of breaking Hulman's unwritten rule.
Last fall, The Star decided to solve the mystery.
A six-month inspection of each grandstand resulted in a count of 257,325 permanent seats, which includes the five types of suites scattered around the Speedway. It is believed to be the only formal count of the seats in the facility's modern era.
Wikipedia says
It has existed since 1909, and is the original "Speedway," the first racing facility historically to incorporate the word. With a permanent seating capacity for more than 257,000 people <1> and infield seating that raises capacity to an approximate 400,000, it is the largest and highest-capacity sporting facility in history <2> (by comparison, the world's largest stadium seats 150,000 spectators).
In the 6 years I attended the race as a participant,(1990 thru 1995) there was much talk about how many seats there were and how many attended on Pole Day and on Race Day. It was said the the single largest gate take of any sporting event in the world was the 500. The second largest was Pole Day. Admittedly, since the CART/IRL split, this is no longer the case. Empty seats on Race Day have been quite obvious on TV broadcasts in recent years and Pole Day does not attract nearly the crowds it did in the 90's.
Two of the heaviest attended races when I was there were 1992 & 1993. I remember standing in Pit Lane and looking to my left toward turn 1 and the right down the 5/8 mile front stretch to turn 4 and seeing the stands full. On
Pole Day. Plus another 100,000 or so in the infield.
The 300,000 figure I gave was from memory, and that was, if I'm not mistaken from another informal survey ordered by Roger Penske. That survey came up with a seating capacity of over 300,000 and with infield standees and the cars allowed inside, estimates for Race Day have been as high as 510,000.
So...my apologies for asserting you were wrong. You weren't, but nobody really knows. And that's the way the Speedway likes it.