How much weight can i lose on a 13 day water fast if im 5 feet 6 and 121 pounds?

Almost impossible to quantify. Depends on your metabolism and activity. I assume you intend to consume nothing but water for 3 weeks. I would do a little more research into the potential risks to your health before embarking on this mission. You risk malnourishment, changes in mental state, hypoglycaemias, and kidney damage.