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How is the nuance of each one?

  1. Do you need help?
  2. Do you need a help?
  3. Do you need any help?
  4. Do you need some help?
  5. May I help you?

And which one should I use to offer some strangers my help in stations etc.?

4
  1. Do you need help?

This is used for directly asking if a person needs help without thinking whether the person actually needs help or not.

  1. Do you need a help?

This is not grammatical. "Help" is not a countable thing, so an article "a" shouldn't be used. (I learned this from @user3169)

  1. Do you need any help?

This is used for offering help when an asker is not quite sure if a person being asked actually needs help.

  1. Do you need some help?

This is used for offering help when an asker is pretty sure that a person being asked needs help.

  1. May I help you?

This is kind of formal, something you would be asked by a staff at some store or by an operator of some company on the phone.

It's probably not a matter of one being better than the other. I'm sure native speakers are choosing which one to use intuitively (even though I'm not 100% confident in the definitions I gave you in my answer...)

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Do you need help?
Do you need any help?
Do you need some help?

are equivalent in asking if someone would like assistance, and is usually used after observing someone in distress.

May I help you?

is formal and polite and can be more proactive by the speaker (depending on context).
It is asking for permission to provide help.

Do you need a help?

is grammatically incorrect, however you could say

Do you need a hand?

which means the same as

Do you need some assistance?

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  • Then, is there no slight difference in the nuances between "help", "any help" and "some help" at all? – Taiki Bessho Jun 22 '16 at 8:08

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