The circle teleports "any creature who stands on it" to a solid object. My reading of the phrase "any creature who stands on it" is that you have to end your movement on it to trigger the teleport. How many people can end their movement standing on the circle in one round? MrLemon has done the math already and came up with 480 people per round.
However, note that all those people are being teleported to the same spot, and it's going to be six seconds before they get another chance to move. The teleportation circle stops working if you attempt to teleport people "into a solid object". How many people can we pack into the destination point before it's full of people and the spell starts failing? We can probably only teleport eight per round, assuming they're all squeezing.
I suppose you could set the destination to a point above the ground, so that the people being teleported would immediately fall and the destination would stay clear. But then you're dumping up to 480 people per round onto a giant pile of people, with consequences ranging from uncomfortable to fatal.
So: my answer is you can teleport eight people per round safely, or "as many as can reach the circle" if you don't mind dumping them out of the air in a giant heap at the destination endpoint.
If you really need more than that, consider casting gate instead. The duration is shorter (one round per level, requires concentration) but people can move through it normally without the need to stand somewhere and be teleported.