The description of the Deck of Many Things (DMG p. 162) states the following:
Before you draw a card, you must declare how many cards you intend to draw and then draw them randomly [...]. Any cards drawn in excess of this number have no Effect. [...]. If you fail to draw the chosen number, the remaining number of cards fly from the deck on their own and take Effect all at once.
So you have to decide on a number of cards you want to draw and then do so.
My question is:
What prevents me from simply declaring that I want to draw a single card, get its effect, then deciding if I want to risk another card?
If there is nothing that prevents me from doing so:
What benefit would it have to ever declare that I want to draw multiple cards?