Please don't let the dealer rape you like that
I'm with NorthStyle ... why is the dealer suggesting you replace the throttle bodies for low idle
I might consider trying another dealer or a competent shop that is familiar with the 370Z.
To answer your questions, yes there are two throttle bodies. They are very easy to identify if you open the hood. No clue on the pair replacement claim - I'd be hesitant to believe the dealer though based on the price gouge. That price is completely unreasonable. I think you can do the work in your driveway (or parking space) with a just flat head screwdriver to remove the intakes and a proper sized torx bit to remove the throttle bodies. I would expect replacement to take about 2 hours for someone that has no experience. And I'd rate the job difficulty somewhere in the 3-4 range out of 10.