Does playing a "Double Rent" card in Monopoly Deal count against your card play total on a turn?
It counts as two cards out of your three for that turn. Taken from the FAQ monopoly-game.net:


  Q - IF I USE A ‘DOUBLE THE RENT’ CARD WITH A STANDARD RENT CARD, DOES
  THIS COUNT AS ‘PLAYING’ 2 CARDS, OR 1? A - This would count as 2 out
  of the possible 3 cards you can play per turn.


Additional sources that confirm this:


MonopolyDealRules.com on rules for Rent cards