I don't believe that we are actually told how many hours there are for sure. But we do know that the clocks are not the same as they used to be.
We learn this in the very first sentence of the book where we hear that the clock is striking 13. From things we are told in the rest of the book, it seems likely that the clocks have 24 hours on their faces. We are told that Winston and Julia have an old-fashioned clock that has just twelve hours. And sometimes when they talk about the clock, they convert it into what we call 24 hour time -- having 1 PM be 13, 7 PM be 19, etc.