[Stratrat]

I think it's limitless for all practical intents and purposes. Even given something as simple as a 3-chord progression on a guitar, there are countless ways to play it to make it different. Once you get beyond those permutations, consider the inclusion of other instruments (drums, bass, keyboards, etc.) and the differing backgrounds they can provide. Now factor in effects (distortion, chorus, tremolo, etc.) and you can add even more variety. You could have many songs that are quite alike in the technical sense that they're comprised of three chords in a 4/4 time signature - but in listening to them, they'd hardly bear a vague resemblance to each other.

This would be an interesting experiment in the collaboration forum - set out a 3-chord progression - say G, C and D - and challenge people to put together a song using only those three chords. I'll bet every single one of them would be different.