Doesn’t really matter about reps. In my abs blast class I do at lunchtimes we do exercises for a minute at a time and it’s quality not quantity of reps. We’ll rotate three or four different exercises that focus on a different area of the abs and arrange them so that you’re not focussing on the same area on subsequent exercises, that way we transition from one exercise to the next with no rest. So we might do the following sequence: low abs crunches, high abs crunches with knees raised, bicycle crunches, to raise legs and touch toes then into leg lifts or v-shapes. We might do that sequence three or four times with about 15secs rest between then move onto some static work like the various plank exercises (straight plank, side planks and/ or straight planks with one leg raised).
The key to good abs technique is to never relax through the whole exercise. So if you’re doing simple crunches you never lower your shoulders completely and relax your abs, you keep them under tension all the time and just pump up and down.
Don’t forget to stretch them out after, as well as you’re lower back.