That's a pretty broad question but I think I understand where you are going.......you don't want to buy more than you need but you do what "what" you need.
I can't tell you what the "right" number is because it's undoubtedly different for each family.
First, I would start out by thinking about and then deciding on what size unit you are likely to stay in most of the time. FYI, many seem to feel that the starting place is a 1 bedroom. We don't feel that way at all but it's something you should consider.
Second, I would consider what time of year you are most likely to be going to WDW.
Third, consider the likelyhood of whether you would be going every year, every other year, or every third year.
Once you've thought about each of these things you can then come up with a pretty good approximation of what you will need (at a minimum).
Other things you may want to consider include the ages of your children (if you any) and whether you might take friends or family.
All of these things may enter into the question of how many points you need.
Remember is you plan on staying on Friday and Saturday nights then your points requirements increase accordingly (about double).
I hope this helps you organize your thoughts better. Others are going to chime in with their advice and it's likely they will add things that I have omitted.
Good luck