From the Super Mario Wiki
In this minigame, the player has to find one of the four characters (Mario, Luigi, Yoshi, or Wario) amongst a large crowd of the others by tapping whichever of their icons is shown on the wanted poster on the top screen with the stylus. As the player advances, the minigame increases in difficulty, adding more characters, hiding them behind other characters, and even having the icons move. The player starts with ten seconds on the first round, and gains five seconds by tapping the correct character, but loses ten by tapping any other one; the timer has a maximum time limit of fifty seconds at one time. The highest score the player can obtain by clearing the levels is 9999; afterward, the game continues, but the score counter and level number not increase. The minigame ends after the player runs out of time. The timer begins with ten seconds. It can go up to 50.
In multiplayer, the difficulty can be adjusted, and the harder it is, the harder it will be to find that right character. The timer starts at 30 seconds. No time is added or removed. It continues to countdown. If someone taps the incorrect character, that player will lose one point each time. If they are right, they will get one point, and a new character will be shown. When the timer reaches zero, whoever has the most points wins.
 In-game instructions
"It's easy to get lost in a crowd! Find the one who disappeared and touch him. There he is!"