The only legitimate way to get more than one Master Ball in Pokemon Emerald, Ruby or Sapphire is to receive one attached to a Pokemon from another player during a trade. Glitches can potentially be exploited to receive more.
Pokemon Ruby, Sapphire and Emerald versions contain only one Master Ball. The Master Ball is meant to be a unique item that the player saves for a special occasion. Technically, players can obtain more Master Balls from other players in a trade. This requires a player willing to part with the Master Ball to trade the Pokemon holding the Master Ball with another player. Once the Pokemon is in possession, the Master Ball can be removed into the inventory and the Pokemon can be traded back or retained.
Other Pokemon games contain glitches that can be exploited to create more Master Balls. In Pokemon Diamond, Pearl and Platinum versions, the Underground Item Duplication Glitch allows players to duplicate items. This method also requires a second player. To perform this glitch, one player must gift their Master Ball to another player. The player who received the Master Ball should save the game and then quit. The player who gave away the Master Ball should quit without saving the game. When the two players boot up the game again, they should both have the Master Ball.