The sketch card insert has been most common in non-sport trading card sets like Lord of the Rings: Evolution, Star Wars: Clone Wars and Scooby-Doo: Mysteries & Monsters.
Sketch card inserts have usually come one-per-box of trading cards, but some sets like Hulk by Topps came one in 12 boxes, and Lord of the Rings: Masterpieces by Topps come two per box.