Reading it over, I suppose it was kind of confusing... sorry bout that!
it's 12 plait, just like you'd braid 12 plait in roo. However, I'll just clarify:
by "plait", i mean the number of cords that you're braiding
by "strands" i mean the number of strands in each cord.
Cord from ubraid it or artcords comes in 8 or 2 strand. (Actually, artcords also has 16 strand but I'm not sure how big that is). Hitching Post Supply only lists the 8 strand on their website.
The 8 strand cord is actually four 2 strand cords twisted together, like a rope.
When I made my 6 plait reins, I used the full 8 strands for each cord.
When I made my 12 plait reins, I split each 8 strand cord into two 4 strand cords (you just kind of untwist it and try to get each set of four strands to twist together separately. i know that doesn't make the greatest sense, but it's sort of like making two small ropes from one larger one).
Thus, the 12 plait actually uses the same *amount* of mohair to braid as the 6 plait because the cords have half the number of strands. This does not include the core need for the 12 plait.
Hopefully that makes little more sense?