You need at least 15 guests and no more than 20. If you have fewer than 15 guests you will have to drop a character (which may affect the plot), or play a character yourself (not ideal, but I talk about it here), or even ask a guest to play two characters at once. If you have more than 20 guests you will need to create your own, or use one of the free extra characters provided by Freeform Games when you download Vanished in Vegas.