Are you asking for a maximum number of points that should be financed?
If so, I think a lot of people will give you the answer ZERO.
DVC is a luxury item and most feel that having to finance is an indication that you aren't financially prepared to deal with it.
I would add a slight asterisk to that argument. I think financing DVC may still be wise for those who have an established track record of visiting WDW annually or every-other-year. I assume there are many people who can afford to comfortably save $2,000 - $3,000 for a trip to Disney every year or two. Yet these same people may not have $15,000 in disposable income sitting in the bank to use for a DVC purchase.
For those folks, I personally think that buying DVC (and having to finance) still makes sense. Even with paying interest on the DVC purchase, in the long run those people are still going to save a lot of money over paying $2-3K cash each year for their trips for the next 30+ years.
Those who pay cash for a DVC contract will usually find that they break-even on the purchase in about 6-8 years. Adding interest may extend the breakeven to 10-12 years, but in most cases that still leaves 30-40 years of low-cost vacations. DVC also has a high resale value meaning that people could sell and recoup most of their investment as long as they hold the contract for a minimum of 3-4 years. That fact makes it a pretty low risk venture.
As for how many points to finance, I would suggest starting with the minimum (160) and go from there. If you have enough money to finance more than 160, I think you'd be better served buying the minimum and making extra payments so that you don't take 10 years to pay it off. Once the initial purchase is paid off, you can add additional point in increments as low as 25 pts.
If you buy more than 160 points, you'll probably use the excess to plurge on larger rooms and high-priced weekend nights. Sticking with the minimum will force you to economize a bit until you can really afford more.