Writing advice from a friend

I had lunch with a writer friend the other day. He's had a successful career - and continues to do so. I was discussing an idea I had for a TV pilot with him and he said, I like the idea, but let me ask you a question that I hear all the time - what's the franchise?

In other words - the idea I had was unique and sexy, but how do you drive it as a series? Where do 13 episodes - or 32 or 50 - come from? What drives the story in a way that really makes sense? And what makes sense and fits in with your premise? It seems simple, but I'd never really thought of it that way.