There will be a few more Detective Pikachu promo cards revealed over the next couple of days.
Pokemon did not announce any details about the promotion or which stores are participating. Perhaps they will provide information once all the promos are revealed.
Pokemon initially stated there would be 26 Detective Pikachu cards. There are 18 cards in the Detective Pikachu set, three Pokemon-GX in upcoming “case files” (Charizard, Greninja, Mewtwo), and two Detective Pikachu promos in upcoming products. This leaves one unrevealed Detective Pikachu card and two unknown promos.
The Detective Pikachu mini-set will release through various Detective Pikachu promo products starting April 5th in the U.S. and March 29th in some international countries.
Update (Tuesday): SM199 Psyduck has now been revealed!