Let's look at some numbers derived while I was sitting between acts.
There were 14 sections that I counted that had actual seats (chair-like) in the "bowl" and were 20 seats to a row with 22 rows in each. At one point, these were all, or nearly all full. That totals 6,160 seats.
On the field were probably about 4,000 people at one point, being for Rob Zombie and at the start of KISS. The Toyota Park website says it handles 8,000 on the field. This brings the total to 10,160, and this does not
take into account those sitting in the bleacher seats on the curves of the venue, at the very back or in the upper deck of the bowl - just the sideline views in the lower deck and the field.
The Toyota Park website says the lower bowl accommodates 17,500, and the total for a concert is 28,000. Based on the numbersabove, one might be able to extrapolate at least another 10,000 people (more or less) in those other areas of the lower bowl. The lower deck, for Zombie and at the start of KISS, was packed. The upper deck, all day, appeared to have very few people in it. I could only see one side, but if they were both half full on the upper deck, that's another 1,250 since the upper deck is supposed to hold 2,500 in total.
Some people did take a break at Shock Me. Read into it what you want, to me it's the half-way point of the set and most of the people leaving, not being die-hards, are getting thirsty or need to take leak. Most of them came back.
When people started leaving was after Rock And Roll All Nite. It wasn't immediately clear KISS was going to have encores. In fact, I was surprised, given there is likely a curfew, and they probably played well past it. Moreover, looking at tonight's and Sunday's headliners, the shows end around 10:30, so KISS playing that late after their confetti shower was probably confusing to the average festival goer. Add to it the sheer length of the day, and maybe some people decided to go home because they were tired. Some also have the mindset of wanting to get out early to beat traffic. Fair enough, but we got out of the parking lot relatively painlessly.
Field Approx 4,000
Lower Bowl Approx 16,000
Upper Deck Approx 1,250
I would ballpark the attendance number could be between 20,000 to 22,000 on the whole day at its peak, whenever that may have been, give or take a few as I'm only estimating. Certainly not scientific by any stretch, just my personal observation, and of course, as the day went, the crowd grew. Anthrax and Megadeth certainly played to smaller crowds than Zombie and KISS.
BTW This date was competing with One Republic at Ravinia (3,500 seat pavilion and by some estimates in excess of 15,000 can be accommodated on their "lawn") and the Pitchfork Music Festival in Chicago.
For capacity info, see this site, scroll down to "Seating Capacity"....