How do I add another ipod to my account

I have an itunes account in under my name. I have purchased ipods for my sons and have them all registered under my account. They all sync my list of purchased itunes, but the newest ipod I bought is not syncing my itunes list. What is problem? I undersood I can sync up to 5 devices under my accout. How can I see a list of devices under my account? I may to delete one device to add another. We have 3 nanos, and 2 ipod touchs. The newest ipod touch is a replacement purchase for a lost or stolen device.