Multiple Airport Express

First world problem here...Using a PC running Itunes, I can "output" one song to up 5 airport expresses. I use this out on the lake with my boat and friends boats. So I can have my boat and 5 others playing the same song at the same time. Is there a way to expand this so I can send to more than 5 other devices? Any suggestions would be greatly appreciated. As an added bonus, I would love to not have to hook up my PC to make this happen.