Nah there's more than 20. The rank exists in all services except the Air Force (they got rid of it) and the Coast Guard (they never implemented it).
So between the Army, Navy, and Marines there are definitely more than 20.
And yes they know their stuff, they are awesome at what they do.
I think I remember seeing somewhere around the time of my EAS there was like 12 or 13 in the Corps total.
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