How do I testfor a certain amount of players 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.

Using /testfor in a command block while using r= for your radius. To test for a count with no more or no less you'll have to use dust equal to that ammount.

• testfor @p[r=radius, c=maxCount] Circular radius centered on the command block

• testfor @a[r=radius, c=maxCount] (Will match dead people supposedly)

• testfor @a[x=px, y=py, z=pz, r=radius, c=maxCount] Radius from px,py,pz

Needless to say, you will need a clock. (I know there are faster clocks) The first commandblock (lower left) is your testfor, and the upper command block would be your tp @p[x=px, y=py, z=pz, r=radius, c=maxCount]

The blocks in my left toolbar are the materials used for the commands setup, and on the right toolbar are for the clock.

You can easily use PLAYER ARGUMENTS. To do it in your case, you should put a command block with this command inside:

/testfor @a[r=35,c=12]

Then, you have to put a comparator next to the Command Block. Done!