I'd like to add that there is nothing strange about this sentence, too. What if I write it as "Είναι μόνο μία ομάδα"; Then it is a word to word translation to "There is only one team". Greek syntax is much more flexible, though, and wanting to emphasize the word "team", one puts it in front.
Thanks Troll! I think I have a better handle on this sentence than I did four months ago, and I realise now that the μία is an article. Having said that, if I was asked to provide the Greek for 'there is only one team', with no prior knowledge of this sentence, I'd write "υπάχρει μόνο μία ομάδα".
Sorry for taking so long to see this!
This is actually a kind of fixed phrase. Some people are pretty... passionate about the team they support, in any kind of sport (but football is the most common of cases.) So, Ομάδα είναι μόνο μία has the meaning of "each person can only support one team, as it is very special and unique to them."
(I do think this phrase was a twist to the actually well-known phrase Μάνα είναι μόνο μία. Nova's novasports made it a thing https://www.youtube.com/watch?v=7B0HEtmoLOk .)