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She reply me back my boyfriend’s birthday, I was confused if she have more the one bf is the sentence is correct??

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A: Whose birthday is it?
B: My boyfriend's (birthday).

(It is not necessary to repeat the word "birthday", but it is not wrong to do so.)

She has only one boyfriend. boyfriend's is the possessive form of the singular noun boyfriend. The plural noun is boyfriends and the possessive form of the plural noun is boyfriends'.

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banana red 890is the sentence is correct?

Yes. In the very unlikely case of more than one boyfriend with birthdays on the same day you say

It's my boyfriends' birthdays.

CJ