There should be a hyphen after "erme."

An "erma" is a type of bust portrait that includes the head and some of the body, as per:

http://dizionari.corriere.it/dizionario_italiano/E/erma.shtm...

"Scultura che rappresenta una testa umana con parte del busto
sec. XVI"

The term seems to be rare, and certainly in this formulation.
So this is a ring or crown of these cherub sculptures in bust format. You might be able to get away with just saying a ring of cherubs, for that matter, which is why I put part of my answer in parentheses.

I'm not sure whether this is the best formulation, though. There is possibly a tighter way to phrase this.

You can see them here in this photograph:

http://www.arounder.eu/og/Campanile di Sant'Andrea delle Fra...