I took me a while to come up with an explanation with which I was happy and that clients would understand. I tell them it's like going fishing - it's not every trip out that you're going to walk away with a dozen trophy catches. And just like in fishing, I have "size" limits...if we get something too small, we're going to throw it back. I'm not going to hand over a few mediocre shots just to meet some arbitrary quota. That helps no one.
That said, I have always managed to walk away with at least one "trophy" shot from every session. Anything else is gravy, and I make sure my clients know that this is my mindset.
If I go out shooting for a day and get 5 good images I am happy.