Most of the multi-stringed guitars I've made have been classical 8-strings. Typically the lower two strings, the 7th and 8th, are tuned down to D and A respectively, but ultimately tuning depends on the key of the music being played.

I've also made various steel string harp guitar designs (10-20 strings), featuring 4-6 subbasses and as many as 8 higher trebles (above the standard high E). The subbasses and treble bank are played open like harp strings, on either side of the standard 6-string fretboard.