Now there's a fun question! If you and your friends are in a hurry, don't want to get hurt and don't have any buildings nearby, 864 of you can teleport with one casting of Teleportation Circle.
Otherwise you can fit 338304 medium or small creatures with 30ft walking speed through a single cast of Teleportation Circle - Now there's some spell slot efficiency! Most of them will end up dead, but that's beside the point.
The key is to build a 117 story skyscraper above the Teleportation Circle with 1.5m high halfling style ceilings and a 10ft square hole in the middle of every floor above the Teleportation Circle. Why? Well, on the turn when a creature starts falling, it immediately falls a turn's worth of distance.
- Mike Mearls made this neat falling time and speed calculator: https://www.angio.net/personal/climb/speed
- Enter height as 117 * 1.5m = 175.5m and you'll get less than 1 round (6s) of falling time.
The creatures take falling damage (20d6 bludgeoning damage from 41st floor upwards) and then teleport to the nearest unoccupied space at the destination teleportation circle.
Here's the math: The portal closes at the end of its caster's next turn, so everyone has one turn to get in. First they all crowd around the hole on their floor in order of increasing initiative so they won't get in each other's way. Then everyone besides the caster readies a Dash action, which gives them 30ft of movement when a condition is triggered. Let's say the condition is "Having a free direct path to the hole on my floor after Teleportation Circle is cast below it.". After everyone's ready, the caster (one with the lowest initiative on the 1st floor) casts the circle and walks into it (lucky bastard). Then floor by floor everyone Dashes for their floor's hole on the caster's turn. Those within 30ft make it on the caster's turn. Others use the readied Dash to fill all spaces within 60ft of the hole to move and then Dash again on their next turn. This way a total of 101088 creatures would enter the portal - 192 creatures per floor with the readied Dash and 672 per floor on the next round (192 + 672 creatures from 117 floors = 101088).
Why 192 and 672?
- A medium or small creature occupies a 5ft square area.
- On a square grid diagonal movement costs as much as lateral.
- The hole above the teleportation circle is a 10ft square.
- 30ft of movement allows us to empty an area of (35+35)^2 - 10^2 square feet around the hole on each floor. In creatures that's (7+7)^2 - 2^2 = 192.
- 60ft of movement allows us to empty an area of (65+65)^2 - 10^2 square feet around the hole on each floor. In creatures that's (13+13)^2 - 2^2 = 672.
To take the sillyness even further we add another 353 floors to that skyscraper (those will have a height between 177 and 705 meters, as it takes 12 seconds to fall 706m) and fill it with creatures below the caster's initiative, have them Dash into a hole and start falling one turn before the portal is opened, then next turn the portal is opened on the caster's turn just before they splatter through it on their initiative order, adding another 237216 likely dead creatures (353 floors * 672 creatures per floor), just so you can get the most value out of your 5th level spell slot! The Universe will also present you with the "Most dead creatures teleported" achievement and will disconnect you from the overcrowded tabletop game instance =P