In deciding on the number of points, I recommend first considering whether you really think you will need, and use, a Grand Villa, and also, how long in advance do you think you would book.
My husband and I could never even consider booking 11 months in advance. We're lucky if we know 2 months in advance when we can take time off work, which also can occur at any time of the year, but that also enables us to take advantage of off-peak period savings.
So we bought points at the very cheapest Disney property on the resale market, which is Vero Beach, for which we paid a little over $40/point, instead of the current "new" price, which I understand can run as high as $140/point. Meaning that over the next 30 years of our membership, each point costs $1.40/year, while a new membership runs approx. $2.80/point per year (50 years), but our annual maintenance fees run about $100/year higher as well.
Of course, we're always taking a chance on availability, but we can't avoid that anyway.
Depending on the season, the number of points required for a Grand Villa can be more than double the points of a 2 bedroom, most of which can easily accomodate 8-9 people anyway. If you really need the extra room, renting a 2 bedroom plus a studio is almost always a far more economical use of your points.
For example, a 3 bedroom Grand Villa at Animal Kingdom Villas (AKV) during "adventure season" currently requires 545 points, Savannah View.
However, a 2 bedroom, plus studio (both Savannah views) is 220 + 81 points, respectively, or 301 points, a savings of 244 points, which, if you don't use yourself, you could rent out for approx. $10/each, and make $2,440.
At least 1 party in your group might prefer a little extra privacy of that studio, and that is, if you even need that much room, because again, a 2 bedroom easily accomodates 8-9 people anyway.
Most of our family lives in Florida, so we usually spend most of the time visiting and staying with family, and then reciprocating by having them stay with us for a few days at Disney, which gives us even more flexibility on dates and accomodations.