How many Ancient Greek gods are there? I am studying for school, and only know Zeus, Poseidon, Ares, Hermes, Dionysus, Hades, Aphrodite, Hera, Demeter and Athena.
There is no number.
In a couple different ways, in fact!
First, it's important to note that there was no centralized authority on "Greek" religion. There wasn't even really a unified "Greece" in what's often called the Classical era. Each city-state (polis) had its own government, its own language, and often its own gods.
One of my favorite sources on the diversity of Ancient Greek religion is Pausanias, who was writing several centuries later (in the Roman era) but tried to document as much as he could all sorts of different local legends and practices. He spends several volumes talking about local cults (as in systems of worship, not high-control groups) to gods, heroes, and specific aspects not seen anywhere else. For example, he discusses a cult to Medeia's children in Corinth, and the various legends behind it, and an unusual cult to "Weapon-Bearing Aphrodite" (Aphrodite Hoplismene) that probably shows Mesopotamian and/or Semitic influences, and veneration of Sisyphus who might have been a sort of local trickster god before he got syncretized into the broader mythology.
As a side note, this is also why the myths contradict each other so often. In one polis, the changing of the seasons has to do with Persephone's return from the underworld. In another polis, Persephone is queen of the underworld in her own right, and the changing of the seasons is tied to her lover Adonis being shared with Aphrodite in the world above. When we talk about "Greek mythology" we're really talking about all sorts of different belief systems only vaguely unified by language and culture.
So, in one sense, there is no number because there are so many hundreds of local variations. Pausanias documents dozens and dozens of them, and yet this was probably only a small fraction of the diversity out there.
In another sense, though, even the Ancient Greeks didn't think there was a specific number of gods out there! To quote Pausanias one more time, talking about an Athenian harbor (I.1.4):
There is a temple of Athena-Sciras, and another one of Zeus a bit farther away, and altars to the gods said to be unknown, and heroes, and the children of Theseus and Phalerus.
Especially in the Roman period, there were gods out there that nobody knew about, whose identities had been lost to time or perhaps never known in the first place. There have been sacred sites discovered in Rome with inscriptions saying, in very archaic Latin, "whether this is dedicated to a god or a goddess, this altar was restored by Gaius Sextius Calvinus"—even if you don't know which god it was originally meant for, it can't hurt to clean up the altars and treat them with respect!
So, to sum up, there's no real number of Greek gods. There were hundreds of minor gods or variations or aspects of the primary gods venerated over the centuries, and even then, at least some worshippers believed there were other gods who were not (or could not be) known.