Well Hyrule Warriors (Legends) has 25 characters in the base game and an additional 4 as dlc. Given that it is a conglomerate of the original wiiU version and all dlc (if im not mistaken) and using it as a benchmark for Fire Emblem Warriors, I would estimate probably around 20 characters in the base game and up to an additional 10 of dlc. Probably prioritizing ShadowDragon, Awakening, Fates, in the starting roster, with Alm and Celica additionally since they're getting a remake, and three sets of dlc containing the major characters from other games (Genealogy set, Radiant set, Blazing Set). Guaranteed Anna the merchant.