Currently, the second largest known number of the 15¢ sheet stamp on cover, parcel piece, or tag is: 21.
Description: 2 pound 8 ounce air parcel to England.
Owner/Source: Robert Schlesinger Prexie postal history exhibit.
Currently, the third largest known number of the 15¢ sheet stamp on cover, parcel piece, or tag is: 15.
Description: Parcel containing shell casing recovered on Guadacanal and mailed from New Caledonia to Philadelphia on June 13, 1943, and received in Philadelphia in July 1943. Parcel originally contained 16 stamps, but one dropped off.
Owner/Source: eBay auction listing.