Man, I like that one yen coin.

I don't know the answer, but it is srinking quickly due to inflation around the world and costs to support a single unit note. Of course the same will apply to ANY note of lowest denomination eventualy. Many countries are going to coins or polymer plastic. I have expected the US to follow soon. The $1 coin has not been recieved well by the public. Ike dollar, Susan B, Sack-of-Goweea, and the presidential dollars all flopped.

My guess is 10% of countries still have a 1 unit paper note.... and that's a wild guess.