Unit Ownership

How do u create a trigger like
If a player owns a unit, he cannot own another unit of that kind.

How do players gain ownership of units?

by stepping on the circle of power.

Every time this default "unit" steps on the circle of power, just do a check to see whether or not the player already owns one of those units associated with that circle of power. I'm assuming that when you go onto the circle of power it gives you a unit.

use this to detect the condition, then do whatever u like