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?