Why did G-d punish the Egyptians with exactly 10 plagues, not less, not more? What is the significance of 10?
The Maharal in chapter 57 of Gevuros Hashem writes, in one approach, that the plagues were meant to correspond, and in a sense undo in Egypt, the 10 statements with which the world was created (Pirkei Avos 5:1). He goes through the correspondence and how each plague was removing one aspect of creation. So the significance of 10 was its correspondence to the 10 statement with which the world was created.