The Deck of Many Things (DMG, pp. 162-164) says:
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. Otherwise, as soon as you draw a card from the deck, its magic takes effect. You must draw each card no more than 1 hour after the previous draw. 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.
But it doesn't seem to specify an upper limit to how many cards can be picked. Is there one? Because of how dangerous the Deck of Many Things can be, I've only ever seen people pick up to 3, but could you declare 10? 30? 100?