Keep in mind there are different kinds of strength. Maximal strength cannot be developed by crunches or sit-ups. Repetitive Strength can though. You have to mix them up. And like Chris Nagel said, you have to hit the obliques and lower back as well. A strong core is essentail in boxing. Ducking, weaving, taking and giving punches all realy a lot on the core. So remember to work the maximal and repetitive strength of the abdominals, obliques and lower back. Rather than reps try circuits like this would be ideal for beginners for example:

10 knee-to-elbows
10 leg raises
10 crunches
1 min plank

Perform 3 times as quickly as possible (remember quality of course, more like as little rest as possible). Soon you will be able to add harder exercises, or weighted ones. And never forget the obliques and lower back