Code can be anything from text and title screens to music and gameplay. And then there's also "game" in terms of a product, something that has a title different than something else being sold.
So the issue is both in terms of programming and also an actual item/distribution when deciding what an unique game is. Aside from giving one precedence over the other there is the problem of a spectrum inside of them where things differed from one bach of units to another.
Therefor there isn't any specific point. Personally i'd measure gameplay and not sound chips or changed sprites. Return of Double Dragon and Super Double Dragon are unique games to me. But you could always force me against the wall and ask whether a single changed enemy at some spot would constitute an unique game. I'd be forced to say no, but if i were really being 100% consistent with my argument i'd really have to say yes because the gameplay by definition differs.
So to conclude there isn't any objective measures but a subjective one, and on a spectrum at that. Wherever you draw the line of it becoming too petty to care about.