how many paver base bags do i need for a 15 foot by 18 foot area?

That' depends on how deep you are going. If you read the details for the bag, it covers 6 square feet at 1" depth. For a 15x18 area, 1 inch depth of base you would need 45 bags. One nice thing about the bags at Lowe's is they list the number of bags for an area and depth to help you gauge what you might need.

You will need around 170 bags.