There are people who get some kind of kick out of stealing a small bag of pretzels. They likely toss the thing later.

As to what's appropriate? The better question is, "how much do you want?" I wouldn't take more than 2 items without letting the basket make its way through the cabin. If you want more, ask the FA and presuming that there is more to be had, it's yours. The FA will just say no if there's none left for that flight (occasionally short flights are not catered at outstation turns).