Gillette holds 68,756 for football...to be annoyingly precise.
When you take some OB seats and add the GA, you're looking at around 71,000 I'm guessing. That would be a record crowd for a New England area U2 show, if anyone keeps track of that.
Yup that sounds right, I work security at Gillette, and I found an old briefing sheet, I can share some info (that at least applied to a patriots game):
-they expected 70,000 attendance (that must also include media, teams, staff maybe?)
-bags are allowed, but no larger than 8.5"x11"x6" (they are usually pretty lenient about this, usually you just have to try to stuff your bag into a box which is like 8.5"x11" or something, but I don't think the 6" matters that much, as long as it fits half-way decently
-obv. no alcohol, food, coolers, umbrellas, video cameras
-first employees start checking in 6 hours before the game starts, ticket office and will call open 4 hours before game, parking lots open 4 hours before game
I'm sure some of these things might be different for a concert, but hope that helps a little!
EDIT: also, from my experiences, from the third level, people on the field are pretty much pin dots so I would recommend binoculors