1 per hill.
Think of it this way, you are going to be paying $2 to $4 per rhizome. 1 acre can hold 1000 to 1200 rhizomes in most planting configurations. So lets say that 1 per "hill" is now $3300. If you go 2, $6,600. 6 and you are paying $19,800. The guys planting 6 to 8 per hill are planting stuff they have pulled out of their own fields and are paying about $0.50 or less per rhizome to harvest.
On a good batch of rhizomes, we easily get 80% popping out of the ground. so if you double up on the rhizomes, you'll probably get 100% of your hills popping up but you just wasted $2640 planting an extra rhizome in a hill that didn't need it.
On the other end of the spectrum, I've seen batches of crap that only has 10% popping up. If you double up, at best you get 20% which is still a dismal failure.
To be honest, I haven't seen much in the middle so unless you screw something up, either there is little protection for doubling up or little reward as you are doomed anyway.