Hard to say. It would also depend on a number of factors such as awareness of each other, terrain, and political situation. The Aztecs were hardly a rabble, and if you assume they are aware of the Romans' advantages, they would be able to fight them the way that would minimise them and maximise theirs: ambushes, skirmishing, engagements on broken ground etc. It would be an achievement to convince them, given the intense dedication to honour that characterised Aztec military culture, but they did appreciate smarts. In that case, maybe 3:1 could be enough. If they are able to go guerilla (which, Aztecs being Aztecs, is unlikely), even less.