Im cooking 12 pounds of fingerling potatoes in an oven roaster. How long and what temp should i cook them at ?

That's an awful lot to do at once. I would divide them up between 4 half sheet pans. you could roast two at a time in your oven on separate shelves. Be sure to stir them half way through.

Could you do them in batches?
Or half in the roaster and half in another oven-proof pan?

Unless they are in a single layer they will come out steamed. What texture are you looking for?