How many crunches a day do I have to do for them to be affective?

im 5'4, 143 lbs which is a lil chunky for my height. im trying to get my tummy in shape and i want to know how many crunches a day i would have to do for them to actually work? Im on a diet, the smoothie diet and only eat about 1000 cals per day (only up to 600 at dinner sometimes not even that many and the rest is from smoothies, nuts and veggies). so im not sure if that affects anything but yea. my stomach isnt that big but it needs to lose some pudge! thanks =)