"Forever and one, I will miss you..."  

After listening to the song it seems to me that he's using forever and one to emphasize how much he'll miss the person he's singing about.  The word forever means: always, continuously, permanently.  And you don't need to add "and one," but the singer did to add more meaning to his lyrics.

I've never heard people use "forever and one," but you could use it to emphasize things you're serious or passionate about.  For instance, I might say to my boyfriend "I love you forever and one" meaning that I will love him forever and ever. 

I have heard people use "Forever and a day."  So, I'm guessing they're similar in meaning.  Then again, who knows?  This is just my best guess.  But I'm glad you posted this question.  I've never heard of Helloween before.  The song was pretty good.