Board coverage already allows for the gap between boards, which is why a 5.5 inch wide board is counted at 5.75 inches of coverage on a 12 foot length. Order by area and the gap takes care of itself.
Gap depends on how wet the boards are
Pressure-treated lumber from the yard is usually still saturated. It shrinks across its width as it dries, so those boards go down nearly touching and open up to a proper gap within a few months.
Kiln-dried, composite and tropical hardwood boards are already at their final width. Those need a deliberate gap — an eighth to a quarter inch — cut in at installation.
Run the boards the long way, or diagonally
Boards running the length of the deck mean fewer butt joints. Every butt joint has to land on a joist and needs blocking if it does not.
A 45 degree layout looks good and adds about 15 percent to the board count because every board is cut at an angle at both ends.
Hidden fasteners or face screws
Face screwing is fast, cheap and holds well. Two screws per joist crossing, predrilled near the ends so the board does not split.
Hidden fastener systems give a clean surface with no visible screws, cost significantly more, and add time. They also make replacing a single damaged board much harder later.
Picture framing the edges
A border board around the perimeter, running perpendicular to the field boards, hides the cut ends and finishes the deck properly. It is called picture framing and it is what separates a built deck from a nailed-down one.
It needs extra framing underneath — blocking to land the mitred border on — and it adds board footage beyond the field calculation.
Mitred corners on the border are the visible detail, and they want to be cut carefully, because a gap there is at eye level from the door.
Fasteners and the quantity nobody counts
Face screwing takes two screws per joist per board. On a deck of any size that is a number in the thousands, and screws are sold by the pound or by the box of 350.
Hidden fastener systems have their own per-square-foot rate and cost several times as much, in exchange for a surface with no visible screw heads.
Whichever system, the fasteners have to be rated for contact with treated lumber. The preservatives corrode ordinary steel, and a deck built with the wrong screws loosens from the inside over about five years.