10 crunches will do absolutely nothing for your abs, definition comes from low Body Fat %. If you want a strong core, that's a different story entirely. I should say that 10 crunches is like doing 1 push up, pretty useless. Move that up to 150 and you will start seeing dramatic improvements in core strength. Otherwise you could do a three tiered set like I do, with crunches, oblique crunches and leg lifts which target lower abdominals. 3 sets of each exercise with 20 reps per set will be plenty for a beginner. I do 3 sets of 50 each morning.