one exercise is enough. I assume you already do squats, deads, rows, etc. The isolation at the end is really just a "finisher" for the abs, its in no way the main workout.
You could do different exercises on different days though. I isolate my abs twice per week, one day i do ab rollouts, the other day i do renegade rows (ok, maybe not isolation, but its directed at the abs).
And situps is really not the best way to build abs. What is your goal with the abs? wanna build size to get an impressive six pack, or do you want to build them for more "functional" stuff? If so, then more stabilization exercises, like the two i mentioned, or planks, etc, would be your best bet.