Hi Mei Zhen, Short answer, there is no difference. Long answer, "you may need" is more typical and generally more appropriate than "you might need". To express the difference more specifically, "pigs might fly" is a common saying in English. You'd use "might" for possibilities that are more of a stretch - "you might need a large bag" - versus "may" for a suggestion - "you may need something to carry your things".

"May" is a possibility that is factual and "might" is more a possibility that is hypothetical. Despite that, both are basically interchangeable

They're the same. "You may need" is a little more formal, though.

Same thing. "May" is more formal