Should I resend the emails if professor do not reply to my email? So I'm sending emails to professors right now (during summer break) to get letters. However, some just didn't reply at all, some replied to my previous saying "I don't know if my major is related to your field but you are welcome to ask me other questions" (Is this a no?)

So, what should I do next? Should I wait for them to reply to me? Should I resend? Should I draft another email?
Do not send too many emails-- this will undoubtedly annoy a professor who is possibly on vacation somewhere when they return to see 10+ emails from the same undergraduate(?).

As to your second point, that is absolutely a no. Why are you even asking professors with whom you are not close enough that they do not even think that your major is related? 

I feel like you may have more major problems, but I would not send more than a couple of emails at this time. If anything, the fall semester will begin soon and you can approach any professors in person to ask for a letter.