so the big 12-534-6-9+8*9 has to have ten teams

Yes. The Big XII signed a contract with Fox for 2nd tier rights (those games left after ABC takes their pick) that calls for a certain number of games. With only nine teams, they won't have enough inventory to fulfill the Fox contract (especially if everyone claims one game for their own 3rd tier deal--I'm looking at you, Texas), and Fox could void the deal, or, at least, negotiate a reduction (and it would probably be more than a mere pro rata deduction since the 9 missed conference games would be replaced with lesser value non-conference games). So, they have to have 10 teams, and those 10 must be acceptable to Fox, or else they risk a hit in the pocketbook.