I am fairly certain it is for one single bottle (but I don't mind being wrong).
Back in the day, when the Captains Club benefits were handed out as a sheet of coupons, there was a single coupon for a wine bottle discount that you had to surrender when you used it: Captains Club Elite Member Coupon (12 night sailing or more).
The above link is a scan of the front and back of the sheet (including the fine print). Here is a screenshot of the coupon front:
When you look at the language in the current Captains Club brochure, it simply says "Discount on any bottle of wine". The $40 minimum is gone (maybe because they are all priced above $40!! just kidding) so it is different now at least in that respect. Because of the coupon days, I read that as one single bottle, but I would be happy to be wrong about that.
I would also think that if you could buy multiple wine bottles at 20% off thru the loyalty discount, it would make the wine packages less attractive.