You need to understand that commas and hyphens are there for clarity.
There is no sin in placing or omitting commas or hyphens, for emphasis or clarity, if they do not change the meaning of the phrase or sentence.
I came, and I saw, and there he was standing on the ledge of the parapet à la I'm flying in The Titanic, looking down below was 500 feet of cliff, on which the edge of the ancient castle stood, and persisted for the last thousand years.
There has been a movement within the last two decades, to omit as many commas as possible from sentences - a movement which you need not subscribe to. Write with commas in a manner that makes sense and clarity to you.
Compare with: When to use a hyphen to coin a new word and when to omit a hyphen?