SPAC's largest attended performance in its history was by The Grateful Dead in 1985, where a total of 40,231 fans showed up to see the band. Since the show, SPAC limits its capacity to 25,100.
Those numbers are the official numbers. Not the actual number of fans in attendance.
I was at the SPAC 1988 show. It was a very bad scene. For those that do not know, SPAC has a very long fence around it, that can easily be scaled by a person. 1988 was sold out, but many, many fans were able to enter without tickets. It was very over crowed, and a mess. It was my first experience with the "in the dark heads". The fans rushed the ramps to the balconies, overpowering the security guards, and before the show, security guards were tackling running hippies that had climbed the fence, like right in front of us.
After the 1985 show SPAC did not let the GD play there till 1988, after 88 they never let the GD play there again. Great venue, but the GD attracted too many people for this venue and it was sorta impossible to police the whole fence and keep the gate crashers out. I would bet that there was never more people at a SPAC show then the 1988 Grateful Dead Show there.
BTW, the show was pretty killer, broadcast on the radio, great Bertha on that one, one of best bertha solos IMHO.