The TFR isn't based on how many people are there, merely the seating capacity. The stadium could have 10 people in it but if it's
capable of having 30,000 then it qualifies.
There's no foolproof way to find out about TFRs but it's up to pilots to try and have the best information available.
AOPA summarized how to keep track of them, and has a
list of the sporting-related areas.
Sporting events are easy but TFRs pop up all the time to restrict flying around forest fires or high-profile events (usually law-enforcement related). It's hard to keep track of
everything with the type of flying we do. In a 9-10 hours nonstop flying day it's possible a TFR might pop up that we didn't see earlier. In the airplane we have two Garmin radios interfaced with a GDL69 and a Garmin-396 with an XM antenna. All three receive weather and TFR information. We also have iPad software that shows the TFRs. It all helps if we miss something in flight planning. Surprisingly the XM-based information is sometimes more up to date than what ATC has on hand.
We haven't added ADS-B equipment yet, but I understand the ADS-B Broadcast Services will include TFR information too.
In my world we should be able to own and fly Cubs without radios and have fun doing it. That doesn't always mesh with 21st century though.
Now back to fun Disney stuff...