Types of Dinosaurs
About 800 species of dinosaurs have been reported so far.
However, for prehistoric creatures like dinosaurs, finding a complete skeleton is rare. Many cases are identified as new species from a single tooth, and it's hard to know how many valid species there were, including differences due to sex and growth.
According to the Fukui Prefectural Dinosaur Museum website, "Over 1,400 dinosaur species have been named, but many did not follow international naming rules or meet the criteria for being recognized as a new species, and about 600 of those names have become invalid" (as of December 2020).
Over 200 new dinosaur species were discovered between 1990 and around 2017, and it is expected that many more unknown new species are still waiting to be found in Australia and Africa.
According to some calculations, there were about 1000, 1500, or even 2000 species of dinosaurs.
Not all dinosaurs that lived became fossils. And not all fossils will be discovered.
A theory has been proposed that Dracorex is a juvenile (young) Pachycephalosaurus. From their appearance, it's hard to believe they are the same animal species. This is the difficulty of studying prehistoric creatures that are no longer alive.
There would have been morphological differences due to sex (male and female) and physical characteristics that changed as an individual grew.
It is difficult to identify these conditions for extinct creatures from the remaining fossils.
Even as research progresses, it may unfortunately be impossible to know exactly "how many types of dinosaurs there were"
(and even if we could know the exact number, there would be no way to verify it).