Let’s say it’s 2, since as others have pointed out, industry figures often count journeys, and a return is 2 of those. And presumably, that doubles the opportunities for a journey not to be taken.
Interesting that the consensus seems to be that nobody has the faintest idea. Because, like you, my intuition is that it could be quite a lot, particularly for returns. The number of times I’ve finished a visit somewhat earlier than planned and decided that paying for an earlier train to go back rather than wait hours on a windswept and facility-light station was the better option is nobody’s business. Even looking at the number of reserved seats on my regular GWR journeys between Paddington and Tiverton/Exeter it’s amazing how many of them aren’t filled (and yes, maybe they chose to sit somewhere else, but all of them?).