It is hard to know the number of dwarf planets on our galaxy, but I can tell you that in our solar system, there are five (5) recognized dwarf planets information here . These planets' names are Ceres, Pluto, Haumea, Makemake, and Eris.

On the other hand, Wikipedia mentions: "estimates are that up to 200 dwarf planets will be identified when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.
dwarf planets

I recommend the link below were you can read interesting information. NASA has been exploring the universe and this site mentions some facts that you will like to know:
solar system exploration .