Can one do too many readings in one sitting?
Probably.
I have done multiple readings at a party - up to 12 in a single night. (I have never done 12 for myself!) Towards the end I see the same cards popping up over and over again, and have attributed it to some people not shuffling the cards enough and having someone else's reading 'still coming through.' To remedy this, between each reading I usually take the deck and turn the cards upright, then shuffle myself two or three times while I am talking to the next Querent - before I have them shuffle. I also rap the long edge of the deck against the table top to 'clear' the cards.
As far as expending the energy - yes - to do 12 readings in a single evening can suck the energy out of you, even if you are perfectly grounded and drank a gallon of pure spring water! We all use energy differently and once you discover that 8 readings should be your limit, then learn to set the limit for yourself. I would recommend switching decks periodically.