According to the traditional Passover story, ten plagues struck Egypt.
The Book of Exodus describes the plagues as disasters sent against Egypt to persuade Pharaoh to release the Israelites from slavery. Their traditional sequence begins with the Nile turned to blood, followed by frogs, gnats or lice, flies or wild animals, livestock disease, boils, hail, locusts, darkness, and finally the death of the firstborn.
The final plague gives Passover its name: the Israelites marked their doorposts with lamb’s blood, and the destroying force “passed over” their homes. In the story, Pharaoh then allowed the Israelites to leave Egypt, although the narrative says his resolve repeatedly hardened during the earlier plagues.
Ten is the canonical number used in Jewish tradition and at the Passover Seder, where the plagues are recited and traditionally symbolically reduced by spilling drops of wine. Biblical passages outside Exodus do not always list or order the disasters identically, and some scholars note that the number ten is a later traditional arrangement. That textual nuance does not change the standard trivia answer: ten.