We bought 180 at BLT. The minimum was 160 also but we did 180 because we tend to stay around 10 to 12 days and a studio is just fine for just me and DH.
We knew that we would not go every single year and would skip about every third year. That means we can carry over that every third year and it lets us spread the points a bit further.
We personally enjoy Disney more when we have that break in between a grouping of trips, if that makes sense.
So our pattern is Year 1- Borrow around 10 to 20, Year 2 - Borrow around 10 to 20, Year 3 - Do non-Disney vacation, bank around 100 to 120. That means when we do our next round of 2 years of trips, we can do several longer trips or enjoy a higher point view.
BUT if you know absolutely that you aren't going to want to skip a year to allow for banking, you might want to consider more. This just works well for our style of vacationing.