Can you tell me what number of sit ups a day for a six pack?

Realize: A "six pack" come mostly from low superficial body fat levels. It's why many people who are very thin can have a "six pack" without really doing any abdominal exercises. So first, focus on optimizing your weight. Abdominal exercises like planks, crunches and sit ups can make the sections of the abdominal muscles more prominent if your body fat is low but there is no magic number.