Question: John 19:33 says that during Jesus' crucifixion, the soldiers didn't break his legs because he was already dead. Verse 36 claims that this fulfilled a prophecy: "Not a bone of him shall be broken." But there is no such prophecy. It is sometimes said that the prophecy appears in Exodus 12:46, Numbers 9:12 and Psalm 34:20. This is not correct. Exodus 12:46 and Numbers 9:12 are not prophecies, they are commandments. The Israelites are told not to break the bones of the Passover lamb, and this is all it is about. And Psalm 34:20 seems to refer to righteous people in general (see verse 19, where a plural is used), not to make a prophecy about a specific person.
Response: There are two keys to understanding John 19:33 in my opinion:
1. John does not use the word "prophecy." He is not necessarily talking about a prophecy. He does not identify the event as a fulfillment of prophecy. Instead, he indentifies the event as a fulfillment of scripture.
2. Prophecies are not the only things that can be fulfilled. In modern English, for example, it is not uncommon to speak of contracts, obligations, mandates or requests as being "fulfilled."
I agree with you that "Exodus 12:46 and Numbers 9:12 are not prophecies, they are commandments." But commandments, like prophecies, can be fulfilled.