I think we're all making the same point, but I thought I'd try and illustrate it with real numbers (however badly estimated they actually are). If anybody wants to correct or improve them please feel free.
The circumference of the first row at Michigan Stadium is probably around 400 metres.
The distance from the back of a seat to the back of the seat in front of it is probably around 75 cm.
This means that the circumference of each row is about 4.7 metres greater than the one in front of it.
The width of each seat is probably around 50 cm.
We can calculate that there are (C + 2.Pi.D.N) / W seats on each row, where:
C is the initial circumference
D is the distance between seat backs
N is the row number (starting from 0)
W is the seat width
At Michigan that becomes (400 + 4.7 N) / 0.5 seats on each row, meaning 800 on the first and 1655.4 on the last. Adding them all together makes the total 112,948 seats, which isn't far off (I didn't take vomitories and walkways into account) - introducing a factor of 0.94 to the formula gets you close enough to the real figure.
The circumference of the first row at Rungnado Stadium is probably around 600 metres (there's a lot of room around the track). If we assume the seats are the same size as at Michigan, the formula becomes 0.94 x (600 + 4.7N) / 0.5 - a total of 136,121.
Some people are suggesting that the seats might be smaller and closer together than they are at Michigan. If we just take 5 cm off each measurement, the formula becomes 0.94 x (600 + 4.4 N) / 0.45, making the total 149,580.
Of course, I haven't taken into account the fact that Rungnado has two tiers - you'd have to use my formula for each, if you knew the dimensions of the upper ring...