Let's start with the "traditional" jeli ngoni. this is what they describe on kora-music.com
Kora-music.com wrote:short string Dendiourou, the highest string, away from the player, plucked with right forefinger outward and inward
long string Diali Diourou, one fourth above Badiourou, plucked with right forefinger outward, shortened with left fingers
long string Badiourou, the deepest string, plucked with right thumb or forefinger outward, shortened with left fingers
short string Kodiourou, deeper than Dendiourou, close to the player, plucked with right thumb or forefinger outward
Think of this as the "basic instrument" probably the oldest configuration, and with this you could play the traditional repertoire. There are many variations on this design when you get out there looking at instruments.
My ngoni came through drumskulls. In addition to the above named strings I have an extra "bass" string which can not be "shortened with the fingers" it sounds lower than the Badiourou and it is placed "closest to the player" physically above Kodiourou. Additionally, after the Dendiourou I have two more strings, tuned above the Dendiourou, when play in succession they make a short scale fragment.
If you were to look at someone holding the instrument it the strings would lay out like this
Bass (not named, plucked like a drone string)
Kodiourou (plucked, but not fingered with the left hand)
Badiourou (able to be fingered with the left hand)
Diali Diourou (able to be fingered with the left hand)
Dendiourou (plucked, but not fingered with the left hand)
Extra string 1 (plucked, but not fingered with the left hand)
Extra string 2 (plucked, but not fingered with the left hand)
There might be names for the extra strings, but I don't know them
I find that with playing things off of Djelika those extra strings are mandatory! Just for the range.
Also, with the extra high strings, some strumming affects are available that you can't quite pull off on the middle strings. Also, the extra bass string helps with some 1 - 5 - 1 type bass lines.
Ok, I think that we've just about exhausted my knowledge on this very cool instrument. As with the djembe, cora and bala, there is a lot more information out there. Saddly it is rather difficult to come by! Somewhere in my readings, I have a list of primary repertoire. If I remember correctly there is some overlap with cora and bala repertoire as well.
Speaking of which, there is a chapter on it in Charry's Mande music which would have more info... (my copy is at the studio)

C