This title is pretty straightforward considering there are a bunch (nineteen to be precise) of Katherines who date Colin throughout his life. John Green actually came up with the title before he wrote the book. Listen to him explain it:
"I created this fictional bibliography of books written by a fictitious author, and one of them was called, An Abundance of Katherines and was about an anagrammatic genius. It was just a total throwaway thing; the same fictitious author wrote many other books about many other obscure topics. But that one stuck with me." (Source.)
That's one way to come up with a book title.
We'd also like to point out that the title only mentions Katherine, even though she mainly appears through flashbacks. The thing is though, Colin doesn't even have an identity without Katherine for the first half of the book; all he is is the guy who was dumped by Katherines. We think it's cool that the title hints at that overshadowing too.