It will be stated in your contract. You won't be able to get or negotiate once onboard. In entertainment - the only crew that get single cabins would be Dance Captain, Musical Director, some 'featured' singers, and 'some' guest entertainers i.e. those paid from shoreside or via their agents (not onboard the ship). If guessing from your name you're a piano bar entertainer, you'd probably get a single cabin, but if you're in the ship orchestra, top 40 band, jazz or B band, those almost always share.
And yes - it's OK to ask if the position gets a single cabin, but you really can't demand one. Also, it might happen that you're alone in an inside bunk bed cabin or you may luck out and get an outside with a porthole.