So I've heard that weapons and tools can hold up to 4 unique enchantments, yet I've given my diamond sword 6, and can't get my crossbow to have multishot AND quick-change. Even if I have a book with multi shot, I can't enchant it with a crossbow that has only one enchantment with the Anvil. Is there a limit with that specific enchantment or the weapon?

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See: https://windows-club.com/crossbow-enchantments/

I play on bedrock, I use that enchantment all the time, i use quick charge 3 and multishot 1, then I use fireworks because 3 fireworks can deal around 20 damage all together, maybe it is because you are playing on java, or you must be in creative to combine them.