You'll need at least 50 devil points for 30 extra battles (You technically only need 28) in order to get that last 8 card pieces. As the event gives you 30 dp itself, this might mean you only need to save up 20 dp from elsewhere. 32 dp if you count the APs you need to buy.

Saying this now so folks don't get surprised close to the end of the event...

Edit: Just so people can see all the math, and also consider all the other factors you may run into...

Full event battle per day: 13 battles * 3 each * 2 lessons 5 ad = 83 battles / day

Party Cracker per day: 83 battles * 12 crackers = 996 crackers / day

Total number of free crackers possible currently = 996 crackers * 7 days 200 from achievement = 7172 crackers

7500 cracks - 7172 crackers = 328 = 27 and a third battles missing. (Round to 28, round to 30)

3 battles cost 5 DP. 30 battles cost 50 DP.

Additional calculation on AP:

83 battles / day * 8 = 664 AP

AP per day, roughly: 12*25 rechard 50 ads 50 friends 20 surprise guest 60 Fridge mission = 468 AP

If all the missing AP is acquired using DP: 664 - 468 (30 * 8) extra battles = 436 AP ~ 440 AP = 44 DP.

Accounting that the lesson and achievements also rewards AP: 436 AP - 240 from event - 80(?) from achievements = 116 AP = 12 DP.

This brings the total of DP needed to 32.

Edit edit: Buying the 10 DP for 200 AP once, brings DP total down to 30. There's also level ups, which recharges your AP. And also the 99 DP level up package...