How to check how many enemies are around player unity3d

How can i achive the following?
.Create points around player.
.In these points enemies will fill them, 1 point 1 enemy and so on.
.The rest of the enemies should go idle and wait.
.If 1 enemy dies then the idle enemy closest to the point should get and fill in the empty point.

What I have tried:

Also lets say i have two armys on the scene.

20 versus 20.

I want the NPC’s to attack eatch other and not go after the player unless the player attacks them.

Thank you.