Despite what the covers show, I believe Nodame only plays one instrument (the piano). At least as of where the US manga is (volume 6) I've never seen her play a different instrument. Part of the show is that her piano playing is more of an instinctive talent than a skill. I think they show her with different instruments on the cover of each volume because she's part of an orchestra, but she's the title character. So they just show her with a different instrument rather than showing different members of the orchestra.
It's funny because when I first started reading, I thought she was going to play a bunch of different instruments too.
