I would like to know how common different name suffixes are in the
U.S./World. What percentage of people have a suffix, what percent are
Jr./II, III, IV, V, VI, VII, VIII, IX, X, Etc. I am the fourth and am
especially curious how many there are and how rare the fourth and the
fifth are.

Apart from European nobility (and even then its applied to the title
not the name) it seems to be a peculiarly American habit to do this.
Its as completely unknown here in the UK as I can say, having never
met ANYONE who calls themselves John Smith IV or similar.

I have a cousin who is Bill Gates III (really) what is more he is the
only King living in England, but his title is King Leo of Redonda. You
can find out all about him at www.redonda.org and how he became king
of a tiny island in the Carribean.