Street Fighter X Tekken’s base roster includes 19 characters from each franchise.
That produces a core roster of 38 playable fighters: 19 from Street Fighter and 19 from Tekken. The game uses tag-team battles, so players select teams of two and switch between partners during matches.
The total can be confusing because later releases and platform versions added more playable characters. Twelve additional fighters—six from each franchise—were released as downloadable content for the PlayStation 3, Xbox 360, and PC versions. Some PlayStation versions also included guest characters from outside the two main series.
The question asks specifically about the base roster, not the expanded roster after downloadable content or console-exclusive guests. That distinction is why 19 is correct even though the complete playable lineup in some editions is substantially larger.