You can borrow as often as you need to, up to one years worth of points. However, you can only borrow if it is needed to complete a reservation, so you can't "make a reservation" then borrow, the points will be borrowed at the time of the reservation, if needed.
And remember, the ponts need to be in the accoiunt for the time frame of travel, not necessarily the date of reservation. In other words, if you make a reservation in your 2009 use year for a stay in your 2010 use year, the points need to be available in your 2010 use year, not the 2009 use year.