Opinions vary wildly on that. Unfortunately, surveying technology is inadequate. There was a survey my college buddy was involved with 12 years ago in Azerbaijan which underestimated the amount of oil by a factor of nearly 20. Qatar's estimate of its natutal gas reserves was wrong by a factor of 40 (they now are confirmed as having the largest reserves on the planet)
The facts are that at no time could ANWR produce enough oil at any one time that would significantly influence the oil market. Furthermore, the ANWR oil will not be for the exclusive use of the U.S. but instead will be sold on the world market.