You draw just two cards no matter how many opponents you have.

If an opponent has just one card in hand, that player discards just one card. Other opponents still discard two if able.

Each player loses 2 life, including you, regardless of how many cards they draw or discard.

As Bad Deal resolves, first you draw two cards. Then the next opponent in turn order (or, if it's an opponent's turn, the opponent whose turn it is) chooses two cards in hand without revealing them, then each other opponent in turn order does the same. All the chosen cards are discarded at the same time. Then each player loses 2 life.

In a Two-Headed Giant game, Bad Deal causes each team to lose 4 life. Your teammate doesn't draw or discard any cards.