We sailed on Serenade of the Seas this past March...one bottle of wine per over-21 adult in each stateroom, max of 2 bottles per stateroom. Needs to be a 'regular' 750ml bottle, no oversize, no boxes. Must be carried on board, not packed in any bags.
We put our two bottles in a neoprene bottle bag that held 2 bottles...at security screening they pulled both bottles out to check them.
The really nice thing about RCL's policy is that this wine brought on board is not subject to any corkage fee, even if you bring it in to the dining room (which we did). Our serer happily opened it for us and stored what was left to drink the following evening.
To make it simple (because we did plan to drink one of the bottles in our stateroom) we purchased wine with screw-caps. No corkscrew necessary (and many wine experts will now tell you that screw-cap wine is often better than corked anyway!).