And, you can only borrow the number of points needed to complete your reservation.
So, if you are making a reservation and need only 20 points to finish it, that's all you can borrow for that trip. You can't just call and say, "I'd like to borrow some of next year's points".
You can borrow as many times as needed, though, so if you are booking several trips, you can certainly borrow points for all of them.
Just remember, that once borrowed, they can not be returned and will expire at the end of the UY they were borrowed in to, if you were to have to cancel a trip.