Tens of thousands have been found over the years. In archeology they are generally called middens. For a lot of cultures studying their garbage is one of the few ways we have of understanding them at all. A lot of pre-literate societies really haven't left much more about them.

But one of the most important middens may be Oxyrhynchus in Egypt. The town was a scribal settlement, and so their midden was filled with literal tons of scrap papyrus. Since the late 19th century various groups have been collecting and translating the papyrus. It's turned into a treasure trove of Eastern Mediterranean documents, from Greek poems and plays to early Christian works. Perhaps to modern historians, what may be more interesting are the more "mundane" stuff, personal letters, shopping lists, tax records, and the like. The Oxyrhynchus midden has provided invaluable information on what life was like in Egypt under the Ptolemys, Romans, and Byzantines for over 1000 years. The volume of material is vast. Not all of the papyri even from the 1880s digs have been translated.

Oxford has been publishing a new volume of material every year or two for over a century, and evidently they still have enough material for decades of future publications. That's with the site not having been fully examined. Oxford has a dedicated site regarding getting an assignment to work on the papers, links to the many volumes of published material, and more.

And that's just one school. Oxyrhynchus papyri have found their way to hundreds of universities and other groups. This one trash heap may be responsible for more doctoral thesis over the years than any other collection of documents.

There is a sizable "hill" in Rome (Monte Testaccio) which is almost entirely old broken pieces of millions of olive oil jars. Archaeologists use the site to gather information about the Roman economy at various times.