You really need to study the Vacation Planner Guides that DVC publishes every year to determine your exact vacation plans. They have the
point charts in them. But I can give you some examples...
We are a family of 3 and booked with points for a 4 n Wonder cruise this July, which is a peak season. We booked at 16 months out, right after the schedule was released and were able to snag one of the coveted "secret porthole" cabins. If you are unfamiliar with these, they are basically what most cruiselines would call "obstructed view" cabins.
DCL just books them as "Deluxe Inside" so they're cheaper / fewer points. There are only 6 of them and 2 of the 6 have less obstruction than the other 4 - BUT the others are connecting. SO. They book fast. They are THE CHEAPEST PORTHOLE room you can book and it's costing us 392 points for the 4 nights.
The required points vary widely by the cabin category and the length of the cruise. They also fluctuate a little bit by the ages of the 3rd and 4th persons in the room. I priced it out for a verandah Cat. 6 cabin with MS for points for DH and myself and cash for DS (who will be 11). The same cruise would've cost 406 points for DH and myself and I forget exactly but in the neighorhood of $1,000 for DS. Using all points it would've run 510.
OK, so the same exact cruise in a Cat. 10 in say January would've only cost us only 262 points. But we couldn't go in January (value season) 'cuz we're not willing to pull DS out of school. The lowest "normal" porthole category is a Cat. 9. This is what we would've done if we couldn't get one of the "SPH" rooms. That would've cost us 406 points for all 3 of us in July - for 4 people it would've been an additional 102 - 112 points depending on their age.
Are you ready to hear how many points for a 7 n. cruise? 7 n in a Cat 10 (deluxe inside cat.) in a peak season (summer) on the Eastern cruise would use 234 pts. per person for the 1st two ppl in the room and an add'l 156 - 188 pts for ea. of the 3rd and 4th if between age 9-11. Only 171 - 203 add'l pts. if 3rd & 4th persons are age 8. And only 186 - 219 pp for ages 4 - 7. (2 and under are free). This would all drop dramatically in a cheaper season. Likewise, the "Holiday" seasons surrounding Christmas and Easter would be even higher.
OK, so, to answer your question. 450 points would get your family on the ship in January for a 4 n. cruise in a lower category cabin. If you have visions of a balcony or of a longer cruise you would have to pay some cash for someone....