Q: How much muscle mass can one gain just by doing a lot of sit ups?

A: Very little. Sit ups work your abs and hip flexors which together make up around 10-15% of your muscle mass. Correspondingly, you won't gain more than 10-15% of what you could have potentially gained if you weight trained your entire body. Read the nutrition article to learn about muscle-building diet and the weightlifting article to learn how to weight train your entire body. Then come back and ask me any follow-up questions as needed.

If you do not have access to gym equipment, you can still have a decent full-body workout program by doing pushups, pullups, sit-ups/crunches, squats, jumping jacks and other improvised bodyweight exercises. You can vary a lot of them to make them more intense. For example you can try one-legged squats or do pushups with your feet elevated and/or with your hands close to one another.