The numbered chords are difficult to understand because they're shorthand and the interpretation differs according to the particular number.
X5 (e.g., G5) means that the chord consists of only two notes: the root and the 5th (G5 = G and D only). This chord has no third, which makes the difference between major and minor, so the chord is itself neither major nor minor but can substitute for either. That's why it's called a "power chord". The root is usually doubled an octave higher; the 5th can also be doubled in unison or at an octave. The X5 spelling is different from almost every other chord spelling in that it takes away a note (the 3rd) rather than adding new notes or altering existing notes.
X7 (e.g., G7) means that it's a "dominant seventh" chord: a major chord with a minor seventh note added. The minor seventh is not in the scale of X major, rather, it's a whole step below the root. In a G chord, it would be F rather than F#, hence G7 = G, B, D and F. If you used a major 7th instead (F#), it would be a GM7 chord, also spelled Gmaj7 or Gma7.
X6 (e.g., G6) means a major chord with the sixth note added. So G6 is G, B, D and E. (E is the sixth note in the G major scale)
X9, X11 and X13 are like dominant seventh chords (X7) but with more added notes. That means that, in pure form,
X9 has 5 notes: root, 3rd, 5th, minor 7th, 9th.
X11 has 6 notes: root, 3rd, 5th, minor 7th, 9th, 11th.
X13 has 7 notes: root, 3rd, 5th, minor 7th, 9th, 11th, 13th.
Note: the 9th is the same as the 2nd note of the scale, the 11th is the same as the 4th and the 13th is the same as the 6th.
Fortunately, you don't really need all of these notes. On ukulele, you can play only four of them anyway. Which four?
In an X9 chord, you must have the 7th and 9th (2nd).
In an X11 chord, you must have the 7th and 11th (4th).
In an X13 chord, you must have the 7th and 13th (6th).
If you don't have both of these notes, you have a different type of chord.
The next most important note to have is the 3rd, because it tells you whether a chord is major or minor. But it can be omitted in favor of another note.
The least important notes, odd as it may seem, are the root and the fifth, though a chord will usually have one or the other.
In an 11th chord, you seldom will have the 9th, and in the 13th you'll seldom have either the 9th or 11th, unless the chord is particularly dense.
What this means is that there can be several different note combinations for an X9, X11 or X13 chord. To take a simple example, G9 could be:
B, D, F, A = 3rd, 5th, 7th and 9th (no root/G - most common uke form)
G, B, F, A = root, 3rd, 7th, 9th (no 5th)
G, D, F, A = root, 5th, 7th, 9th (no 3rd; can be either major or minor [Gm9])
It would even be possible to play a more "incomplete"/"devolved" form like B, F, A, which has neither root nor 5th. The most often encountered 11th or 13th chords are similar to the above forms, just substituting the 11th or 13th for the 9th.
You'll also see Xadd9, which is different from X9 in that it doesn't have a 7th; it's just a major chord with the 9th (2nd) added. So Gadd9 would be G, B, D, A. Similarly for Xadd11 and Xadd13.
You never write Xadd6 because that's what X6 means. Xadd13 is essentially the same thing.
You can make minor versions of all of these chords (except X5) by putting "m" before the number: Gm6 = G, Bb, D, E; Gm7 = G, Bb, D, F… The "m" doesn't apply to the number, it just makes the 3rd minor (a half-step lower). In other words, "Xm" means the minor triad, and anything after that modifies or extends it.
