I do about 400 crunches a night and 100 in the morning. Will I lose weight that way?

The short answer is yes you will, but its not safe and it will take a long time.

Try adopting a more rounded weight loss plan. Say, walk 30 minutes everyday, reduce your calorie intake. To this you could add crunches everyday, but perhaps 50 at night and 50 in the morning - but not the 500 a day.

Assuming you're an adult. If you're younger then its best you speak to your gym coach etc.