just curious question when you do a tournement do you only use the same deck for whole thing? or can you use different deck against different people

The general idea is that you pick one deck and stay with it for the tournament. Switching decks, or even altering the one you're playing with, are not allowed. Many tournament organizers will demand decklists in advance, to be able to check if your deck hasn't been tampered with. At big events, the top decks might even be checked before the final rounds are played.