The Daytona 500 "event" is always the biggest one to join (for some reason). Last year there were roughly 1,000-1,100 that joined. It resulted in a crash, but only lasted about 10 minutes. We got no "restarts" or anything like that, so some people were laps down before they even entered the server.
The iR staff said they had fixed the problem and it wouldn't happen again (That was stated last year). This year during the Rolex 2.4hr race, there were about 900+ that joined. Everything bogged down, but no crash occurred (at least not on my end). So some might have thought this issue was fixed.
Now, 1600+ for one event. Plus they have the Premier IndyCar league. Any type of combination will result in this. Down.