The term "one piece" is what the series refers to as the treasure left by the previous Pirate King, Gol D. Roger. If you are referring to 'what is the treasure', then nobody knows yet. It is...
Neither. For a start, it should be you're (short for you are), not your. But more on point to your question, it would be:What if you're one of the pieces...""one of" signifies there is a plural noun...
Just before his execution, the pirate king, Gol D. Roger announced that all his treasure had been left at the end of the grand line, 'in one piece'. Since then, the treasure's existence has been...