How many calories do you burn purging?

None purging. Purging isn't exercise where you burn calories. You are vomiting up food before it gets to be digested. This also brings up stomach acid which erodes your esophagus and teeth while causing electrolyte imbalances which can lead to weakness and cardiac arrest.
What kind of purging. Purging with laxatives or vomiting or excessive exercise is unhealthy and may be a sign of an eating disorder. See your doctor for help.