Don taught me this the other day and I thought I'd share this, since I find the concealment counter handling non-intuitive.
If you have a stack of units selected and you choose to conceal them, you will get one concealment counter for every unit. In order to prevent this, you have two options:
1) Only select the top unit in the stack and conceal that one.
2) Deselect the stack by clicking anywhere on the map, then right-click the stack and choose concealment: everybody concealed and only one concealment counter on top.
If you have a stack of units selected and you choose to conceal them, you will get one concealment counter for every unit. In order to prevent this, you have two options:
1) Only select the top unit in the stack and conceal that one.
2) Deselect the stack by clicking anywhere on the map, then right-click the stack and choose concealment: everybody concealed and only one concealment counter on top.