SDL offers touch input, on platforms that support it. It can manage multiple touch devices and track multiple fingers on those devices.
Touches are mostly dealt with through the event system, in the SDL_EVENT_FINGER_DOWN, SDL_EVENT_FINGER_MOTION, and SDL_EVENT_FINGER_UP events, but there are also functions to query for hardware details, etc.
The touch system, by default, will also send virtual mouse events; this can be useful for making a some desktop apps work on a phone without significant changes. For apps that care about mouse and touch input separately, they should ignore mouse events that have a which field of SDL_TOUCH_MOUSEID.