The base roster of Super Smash Bros. for Wii U and 3DS contained 51 characters, excluding downloadable fighters.
That figure refers to the fighters included on the game disc at launch across the two closely related versions. The roster combined returning veterans with 17 newcomers, including Villager, Wii Fit Trainer, Little Mac, Greninja, Shulk, Mega Man, Pac-Man, and the Mii Fighter.
Seven additional fighters were later released as paid DLC: Mewtwo, Lucas, Roy, Ryu, Cloud, Corrin, and Bayonetta. Adding those seven to the original 51 produces the commonly cited total of 58 playable characters. That is why roster lists sometimes show 58 instead of 51.
Another source of confusion is the Mii Fighter. Mii Brawler, Mii Swordfighter, and Mii Gunner share one roster entry in these games, even though they use different fighting styles. The 51-character figure follows the game’s standard on-disc roster count and excludes all post-release DLC.