I've stated that this feeds one. Obviously, you do the math, and adjust the above ingredients for the amount you are preparing. Pretty easy!
This is one of those times I would rather use metal skewers. The potatoes tend to be heavy, so the weight is better handled with a metal skewer that will be easier for you to grip.
The potatoes can be par-boiled ahead of time, and set tossed in the butter mixture. They will quickly warm up once on the bbq.