If I understand your question correctly you're asking about planning to have more than enough troops to accomplish a given mission?

The only thing I ever heard was that we should have a 3 to 1 advantage in attacking a prepared defense. Beyond that, I really don't know.

As far as organizing small units with a built in redundancy.....it seems to me that the Army doesn't. Instead they try to get by with the bare minimum. And often in times past when the Army wanted to stand up additional big units, or was looking for manpower savings, the squad was the level that had to learn to make do with less.

This last go around the squad came out intact though. Instead, the brigade took the hit. The Army wanted more brigades so they got more brigades..... but now brigades have fewer maneuver battalions.

It seems to me that redundancy planning for the Army usually happens at higher levels. For example, many people think that Schwartzkopf didn't need as many divisions as he asked for to kick the Iraqis out of Kuwait. But he made sure that he had more than enough before starting an offensive.