Depending on your overall expectations, you can certainly get by with 12 in Legion. You won't be getting 20-25+ kills in NQ chambers or anything but 7-15 kills is certainly feasible. I've personally been on runs where we cleared Ki/Muru/Im Wave 3 and beyond with 12 or less (I think 1 run we even entered with like 9. Think we entered 3 DD WHM BRD COR THF PLD SCH).
An I've never tried lowman and would be a bitch but no individual mob would be off-limits except for maybe Harpy which you could access via Mul Wave 1
Mul you'd be looking at clearing Wave 1 and then being selective on what your target Wave 2. Wave 3 would be off-limits unless you were an exceptional group I think and would require no setbacks. Then you could possibly down 1 of the 2 mobs.
Basically with 12 no loot would be out of your reach except for Wave 3 Mul depending on how good you were. I'd probably take a set-up of something like 4x DD WHM BRD COR PLD THF SCH x3. Alternatively you could drop the THF + 1 SCH and take 2 RNGs.
EDIT: COR + BRD rotate between parties and do Hastes/status removal etc like Byrth said.