Interesting quantitative take studying chord distribution. The basic takeaway seems to be that if you re-phrase a song into a different key you can play a lot with just a few chords (I IV V), and even more with a few others.
While I would agree that you'll be okay playing any major scale in C major (or whatever other major key choose to learn), playing a song in a minor scale on a major scale just doesn't sound quite right. So, I'd double all of the numbers on their final table to account for learning a full set of major and minor chords.
If you know I, ii, iii, IV, V, vi in C then you'll be able to play tunes in A natural minor. Add III to cover A harmonic minor, and II to cover A melodic minor
In real life though the key usually depends on the singer's vocal range, so you won't really get away with learning just one key
In A harmonic minor the III is already a C, you can make it augmented but that is not very common. You generally change the v to a V and keep the VII diminished in both cases where as it is major in the natural minor.
He was saying that the III of C major is the V of A minor, and so if you are planning to play in A minor using a set of chords pulled from C major, you may want to add a III alongside the iii.
Yes, that's a good point in the case of six keys (I think my point holds for just learning three), but it is not the one made that the article seemed to make.
This is only partially true. The chords in A natural minor will be the same but it's much more common to hear a dominant fifth using the harmonic minor so you would change the Em to E especially in classical and jazz music.
nit: jazz is the melodic minor (raised 6th as well). Such a beautiful sound; and the modes allow you to get the altered scale (7th mode of melodic minor) which sounds good(?) over the dominant
While I would agree that you'll be okay playing any major scale in C major (or whatever other major key choose to learn), playing a song in a minor scale on a major scale just doesn't sound quite right. So, I'd double all of the numbers on their final table to account for learning a full set of major and minor chords.