How much? It depends on your definition of NTR. According to
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there is none. Also, the dev’s intentions were clear that they didn't intend any.
Why is it even mentioned then? Because there is one part early in the story where a potential love interest has casual sex, once, with another guy before she has the chance to hook up with the MC. This is explained in the story and is part of the plot development. I seem to recall that viewing the actual sex scene is optional.
Is it NTR? The MC does not even know this has happened so there is no intention by the girl to cause jealousy or hurt. But you know as the player, and some people have called this NTR. So you can choose whether or not to even hook up with this particular girl. If you decide not to, there are two other love interests available to take her place, plus lots of other girls happy to have sex with you. Should come down to whether or not you like the story and the dev's style. I thought it was great overall and recommend you give it a go!