What's the best way to program a "Select this Unit" Mechanic?

In my game the player moves guards around the map using NavMeshes. But, when they click on the map all the guards move at once.

I want a way for the player to be able to press numbers 1-9 and whatever number they press a guard is selected and only the guard that is selected moves where-ever the player clicks.


How would I go about coding something like this? What would be the best way to do it?

I would really appreciate any help given!