there are many theories to training abs,i think like others who share my opinion that we should consider abs as any other muscle and we should train it the same way when it comes to sets and how many days per week,2to3 days per week is all that is needed and 3exercises3sets each is all that is needed.
if you are after thickness in your abs keep the reps low around 10 to 15 and upply resistence,you can use a selectorized machine for that,if you are after cuts and definition do free movements and rep as much as you want [the sky the limit].