In tandem bullet, number of bullet/bullets coming out of the gun is/are:

Occasionally, more bullets are found than there are entrance wounds. This occurs, due to defect in the weapon, or due to faulty ammunition, or with loaded firearm unused for several years. When such a weapon is fired, the bullet may fail to come out from the muzzle. When it is fired again, the second bullet may go off carrying the lodged bullet with it, and both the bullets may enter the body through the same entrance wound. This is called a tandem bullet or piggyback bullet (tandem= one behind the other). The bullets may separate within the body, or before they hit the target. Ref: The essentials of forensic medicine and toxicology by Dr K S Narayan Reddy, 27th edition, Page 202.