Wow, great question. The two well known ones are Pluto and Ceres but there are many more existing outside the orbit of Neptune – these are called Trans-Neptunion objects (TNOs) and indeed there are also some dwarf planets located there. I’d say less than a hundred would exist in total but this is really based on guesswork as there’s a lot of objects which are really big that are far from the sun which have yet to be discovered.
There are two that we (or maybe just I) know of, Ceres (which exists between Mars and Jupiter and Pluto. I would say that there are definitely far more in the solar system out past Neptune that we haven’t discovered yet.