The criteria of being a dwarf planet is:
1) It has to orbit the sun
2) It has enough mass to assume a nearly round shape
3) It has not cleared the neighborhood around its orbit
4) It is not a moon
So based on this criteria, there are five dwarf planets: Ceres, Pluto, Eris, Makemake and Haumea. I am sure there are plenty more waiting to be discovered.
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