Minecraft – Testing for a block in a certain radius

There is the count target selector argument.

/tp @p[x=<X>,y=<Y>,z=<Z>,r=<Radius>,c=<Count>] x y z

To detect if there is 12 players or more, there are 2 main ways to do it. One is simply with a comparator leading from a /testfor @a[x=<X>,y=<Y>,z=<Z>,r=<Radius>], which will emmit a redstone signal strength equal to the amount of players in that area and you can run a redstone wire to activate the teleport command, or do it with a scoreboard counter.

This can be done via the /fill command within /execute.

/execute @a ~ ~ ~ fill ~10 ~10 ~10 ~-10 ~-10 ~-10 pumpkin 0 replace dropper 1

/execute @a ~ ~ ~ fill ~10 ~10 ~10 ~-10 ~-10 ~-10 dropper 1 replace pumpkin 0

You don't have to use a pumpkin, any unused block will do. Put these two commands separately on a fill clock and run a comparator out from either of the two command blocks with the commands in them. Whenever any player is within 10 blocks (including diagonals, unfortunately) of a dropper with the data value of 1 (was that facing down? I can't remember), the comparator will turn on. Whenever they move out of range, it will turn off. That's currently the closest you can get to what you asked.