I don't count private copies because I don't know anything about them.
The main reason for the lower number is lack of data, plus some institutions have multiple copies: Boston have 7(!), NY Met 4, British Museum 3, Tokyo National 3, Chicago Art Institute 3, and many with a couple each.
The main reason for the lower number is lack of data, plus some institutions have multiple copies: Boston have 7(!), NY Met 4, British Museum 3, Tokyo National 3, Chicago Art Institute 3, and many with a couple each.