She had sex with two guys on nov 24, she gave birth to the kid on sept 10. Is it possible for her to get pregnant after have sex on that day or she might sleep with other guys on december to get prego?

Due date/paternity. I would have to know her due date to even approach this question. The delivery date and due date are different. Using her edc should at least give more of an idea about the possible time of conception. However, paternity testing probably is warranted in this situation. Many states now require it in unmarried women who claim paternity. I would strongly consider getting it.
No answer. . There is no way to determine the answer to this question other than a paternity test.