I really don't think there is a limit on number of times you can move 2018 points to 2017 for example, but once you move them they can not go back. You would still be able to bank original 2017 points to 2018 even if you borrowed other 2018 points into 2017. Of course that would be after cancelling the reservation that necessitated the borrowing of the 2018 points in the first place.
And remember, you can only borrow when making an actual reservation. So, as long as you are making a new reservation that requires borrowing, you can borrow the points you need...until, as others said, there are no more points left to borrow!!
Dreams Unlimited Travel is committed to providing you with the very best vacation planning experience possible. Our Vacation Planners are experts and will share their honest advice to help you have a magical vacation.