They could all make it if they hurry.
Let's assume Medium creatures with a speed of 30, such as humans or elves.
The circle lasts one round, so anyone within 60 feet of it (assuming they can Dash) would make it through. A 60-foot radius circle holds 452 5-foot squares (and in reality they can pack together closer than that).
If they don't Dash (because getting 100 people all to sprint at the circle without colliding is really hard) you get a 30-foot-radius circle which holds 113 5-foot squares.
There is going to be some chaos at the destination, because the crowd will get their relative positions and directions of motion rearranged to prevent them from intersecting. Worst case, it places them 5 feet apart and running toward each other, which gives them (assuming a Dash speed of 60) 1/4 second to react before they collide head-on. One solution is for everyone to do diving rolls into the circle to stop their motion, but that only works if you're rescuing a hundred Cirque du Soleil acrobats or something and I'm guessing those guys would have escaped already.
On the other hand, you don't need open space around the destination circle; Teleportation Circle will spread them out as much as needed, even outside the room containing the circle.