How many pounds of potty mix I need for a 14 cubic ft area?

I don't think that conversion can be made with any confidence. Potting mixes will be highly variable in their weights per cubic measure. You will be better off judging by the size of the bag, which may offer a cubic measurement as well as its weight somewhere on the label.

Most bagged soil comes in 1 or 2 cubic foot bags, although I have seen some that were 1.5 cu ft.