First off, you're referring to reps, but the plank is an isometric exercise. For that reason, a single plank might better be seen as a set.
Another view I'd like to offer is that the actual time you do the plank might be more suited to be likened to reps. Rough example of what I mean:
- 120s bodyweight plank would improve mainly endurance -> low intensity (like high reps, low weight)
- 10s plank with 30kg of weight would improve mainly strength -> high intensity (like low reps, high weight)
So since you're going for looks/hypertrophy you should probably be able to hold the plank for ~1min before you add weight. Otherwise you'd train more for short-term strength.