At what temp do i cook a 20 lb turkey

If stuffed, internal should be 170F.

Whatever temp you choose, cook to 170F breast and 180F thigh ... use your thermometer.
If any parts ( wing tips etc) appear to get too crispy, cover with a bit of foil.

Some people like to cook their turkey at one temp , 325-350 the whole time -- others start at a higher temp -- 425 -- roast for 30 min and then reduce the temp to 350 or 325. I prefer the first.

Here's a standard recipe that you can follow for temp and time! http://food52.com/recipes/14669_butter_and_herb_roast_turkey