To be honest. Girls are found intimidating and not as attractive when they have abs. Guys tend to think the girl is stronger than them, and that hurts their masculinity.(So my advise is be in shape, but don't be... to built, I guess..) but about 50 sit ups in the morning and 50 at night would be enough.
Start with 30 to the front, 30 to right side, 30 to the left, and 6 inchs where you hold your legs above ground and shoulders for 30 sec and raise it slowly and do it again. Then do 25 for all of these. Then do 20 for all of these again and so on till u get to 5 and count down by single digits. This is a workout we do for dance.