It depends what game you're playing, but in the 3rd and 4th generation I know you can win a Master Ball from the Lottery Drawings, and in the 5th generation by trading with 50 different people and talking to someone in a Pokemon Center.
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Get rid of some of the master balls! the number of master balls you have is the number of the pokemon
You can't have unlimited master balls although u can give the master ball to a Pokemon and duplicate it at battle frontier
Attach your master ball to ONE POKEMON, next CLONE that Pokemon for how many master balls you want. Next detach them, and release all the clones (if you want)
cheating
CHEAT