The number of players can be set via the options menu ("Gear", then the "Game" tab). Currently you can play against 2, 3 or 4 computer opponents.
To play online against other real players you need the multiplayer version of the game which you can download for free from the official app stores here: Rummy Multiplayer by LITE Games
In the multiplayer mode you play (of course) automatically against several people. Before starting a new game you can select how many people, including yourself, should participate in the game by using the slider.
To change the number of players in offline mode, please tap on the field with the three lines (top left) in the start menu, select "Offline" under SETTINGS and set the number of players here.