G-d used ten plagues (blood, frogs, lice, wild animals, pestilence, boils, hail, locusts, darkness, and death of the firstborns) to punish the Egyptians for enslaving the Jewish nation. Why did He choose to use ten as opposed to using, say, one plague? Or any other amount of plagues?
Rav Moshe Almushino 16th century explanation of Pirkei Avos 5:5 explains
that the 10 Makos are to give punishment for the evil doing (i.e enslavement of the Jews) of sinners who are destroying Hashem's world (i.e the chosen People who serve Hashem) that was created in 10 utterences. As it says in the first Mishna: "Surly Hashem could have created the world with one utterance, But He chose to make it with 10 utterances to give reward to the righteous and to punish the wicked. So the one sin of enslavement caused 10 makos as repricusions.
He continues that the Egyptians were punished a further 10 makos for chasing after the Jews with the intent of killing them at the sea.
(It would seem that Rabbi Eliezer quoted in the Hagada held that the Egyptians did 4 particular severe Aveiros (not sure what they are) and were hence punished 40 Makos, and Rabbi Akiva held that they did 5 particular aveiros and were punished 50 makos. Though th Tanna in Pirkei Avos clearly argues and says only 10 makos occurred in egypt and ten by the sea.)