See this page in: Russian
How many species of dinosaurs were there?
Nobody really knows the answer to this question. Estimates vary widely. We’ve seen figures between 250 and 550 or more (one research estimated a whopping 500,000 species). The numbers will continue to fluctuate. Many of these “different species” were actually assigned separate names based on scanty fossil evidence, sometimes only a tooth or vertebra. Dinosaur fossils are often expensive to excavate and collect. So, relatively few remains have actually been obtained by museums and paleontologists.
We haven’t found any current estimates, but by the end of 1981 (according to an article in the Scientific American), there was only a total of about 5,000 fragments of dinosaur skeletons. 
New fossil discoveries and new studies of previously classified fossils may increase or decrease the number of known species. When skeletons are compared more closely, we sometimes find that animals previously classified into two or three different species are actually one species. Differences in skeletal details may be due to differences between sexes (sexual dimorphism), age, disease, or other natural variation among individuals.Paleontologists have often been confused about how to classify and name certain fossils. For example, many years ago dinosaur expert Othniel C. Marsh invented a sauropod family named Atlantosauridae. Into this family he placed the Brontosaurus, Apatosaurus, and Atlantosaurus. Years later, the leading classification expert A.S. Romer decided that all three of these dinosaurs, plus Titanosaurus, were the same as Apatosaurus. In 1966, he changed his mind and reported Apatosaurus was different, but Brontosaurus and Atlantosaurus were the same. Later authors felt that Apatosaurus and Brontosaurus are the same, but that Atlantosaurus is different. It has also been suggested that the Camarasaurus, Morosaurus, and Unitasaurus are all simply young forms of the Apatosaurus. The Aepisaurus and Aegyptosaurus may be the same as Titanosaurus.
The same type of confusion is found among all the different main types of dinosaurs. For example, it has been suggested that Agathaumas and the Polyonax dinosaurs may be synonymous with Triceratops. Among the Carnosaurs, it is thought that Saurophagus, Ceratosaurus, Creosaurus, Dryptosaurus, Labrosaurus, Laelaps, Poecilopleuron, and Allosaurus may be the same thing-Antrodemus.
At least some of the different “species” of hadrosaurs and ceratopians (particularly Triceratops) may have actually been of the same species. It is certainly possible that at least some of the variations were actually males and females of the same stock. These and many other classifications continue to be confusing and sometimes controversial.
1. Dale A. Russell, “The Mass Extinctions of the Late Mesozoic,” Scientific American, Vol. 246, No. 1 (January 1982), p. 63.