The number of footings is going to be a function of the construction of the deck. Lumber, say, 2x8 pressure treated pine can only span a certain distance. Up that to, say, a 2x10 and a greater distance can be spanned. So the number and placement of footings is part of the larger design of the deck. Those footings also need to a certain diameter and go down at least a set number inches which will depend on where you live in the county and local soil conditions.
I don't work for Home Depot but I believe stores offer a deck design service that will give you plans and a materials list. Understand that plans aren't an instruction sheet, you'll still need to have the necessary skills (or hire someone that does) to build the deck. Your local municipality will also likely require a building permit and inspections.