How are cards awarded on defeating a player in risk?
It's possible the iPad version is using different rules, but the official rules for the classic Risk Game are as follows:


  Eliminating  an  opponent.
  
  If  during  your  turn  you  eliminate  an
  opponent  by  defeating  his  or  her  last  army  on  the  game  board,  you  win  any
  RISK  cards  that  player  has  collected.


So you get all of the cards that the opponent had on hand. This will never be more than 6 cards (which is only possible if you somehow defeated a player who had just defeated another player on their turn). It does occur BEFORE drawing a RISK card from conquering a territory this turn, as your turn hasn't ended yet.

There are two highly-relevant bullet points with regards to this:


If you gain enough risk cards to cause you to have 6 or more cards on hand as a direct result of defeating an opponent, you MUST trade the cards in immediately for reinforcements so that you have 5 or fewer cards.
If you have 5 cards as a result of defeating an opponent and then your additional card from conquering a territory puts you at 6 cards, you trade at the start of your NEXT turn.