This is one of those questions that really needs to be reframed. It is not a matter of how many points one should buy, but how many points one is comfortable paying for in annual dues.
I'm not sure I completely agree with this comment although it is certainly an important factor. I can afford much more in annual dues than we will be paying today.
As we have been Disney guests every year (1 or 2 trips per year) for the last 14 years, our decision on point purchase was based on our anticipated travel plans each year, points required for the accomodations we wanted and resort location, among other things. For us, the financial factor turned out to be less impacting in the equation than it might be for others.
I also appreciate the comments along the lines of 'It is better to want more points than you have rather than of having more points than you want'.
Each person must decide their point purchase based on what is best for them that includes
many factors such as financial viability, vacation time availability, season of travel, preferred length of stay, type of accomodations wanted, resort preference, etc. (I'm sure there are several others I didn't mention.)
Smaller contracts generally sell at a higher price per point than larger contracts although this is not a 100% hard, fast rule. They often sell faster as well, but that isn't a guarantee either. If your future plans put you in a known 'sell' situation XX years from now, it might be wise to choose contracts that have less points each, but total up to the number you are looking for if possible.
The only other comment is that it is entirely likely that your vacation habits and plans will change over time. So... Regardless of the number of points you want now, you may very well want more or less 5,10 or 15 years from now. And if you want less, it should be much easier to sell a smaller point contract, plus that allows you to keep SOME points from other contracts if you don't want to leave
DVC entirely.
Just my $0.02.