How much of it was real?

At a glance, it may be hard to argue that the events that unfolded in Hill House were completely normal and that most of it could be explained with science, but, one must keep in mind that not once were any real ghosts revealed. The constant chills they would get could simply be the same chills people get after they read any horror novel. The pounding may simply be trees scratching the window(as we saw was the case for Arthur) and the voices may very well be in their heads. This is actually a very likely case for Eleanor. We quickly find out that she is not quite mentally stable can deduce that voices in her head may be possible. By the end she becomes completely deranged and her narration and judgement cannot be trusted. as for Mrs. Montague's divination, it is never certain whether she made it up or not. Certainly we can see that she is a staunch believer of the supernatural. Any suspicious activity would lead her to point at the supernatural.