how to estimate wood order
Your drawing is showing a staggered pattern in the boards.  Is this your intent?  If so, you'll need something to screw down to at every butt joint.  I see one joist at the end but nothing else to hold it up.  Next, you won't find many mills who will "cut to length", so every 2x6 deck plank will be 6' or 8' long.  It's easier to put down the 8 footers, then snap a chalk line on your edge and trim them all down with a circular saw at one time - a good saw blade is required here.  

While on the joists subject, I am NOT a small guy, and 6' between joists seems quite far apart for a 2x6, at least for my size (6'4" 326 lbs.)  Go to the lumber store, pull a 2x6 beam and a pair of 4x4s to lay it across, and measure how much it deflects when you stand on it, and then have someone else step on it momentarily.... (Again, I am not the guy to stand on a 2' cantilever edge of a deck, but that's up to you and code enforcement for your location.)

You may want to consider having your lumber run the other direction, and put some joist spans every 4 feet on center, reducing your cutoffs, and simplifying the process.  Draw in where you expect the tails of the boards to fall, and get an idea as to what the deck will look like when you're done.  If you don't need it to run away from the house, then I'd go with rotating boards by 90 and drop in joists at the appropriate lengths.

The general rule of spacing between boards is a screw/nail thickness - mostly because they're handy.  Boards shrink and swell with the weather, so they need a bit of gap in them, plus you don't want water sitting down in those gaps, or you'll have rot in no time.  I would use the "actual" wood dimensions and forgo the gap in the math (2x6 == 1-1/2" x 5-1/2" in reality) - it'll only be one to two boards difference at best for a deck this size, and it's not like you will not make a bad cut, find a warped board, etc.

Boards come in 6' and 8' and up.  The longer, the more expensive per foot, as it's harder to cut a 16' straight board than an 8' one. 

Let's say you want to do north/south, in your drawing.
For your short section (using 8' beams cut down to 7'):
182" / 5.625" (I put in 1/8" for gap) = 32.33, aka 33 boards minimum.

Long section 168" / 5.625" = 29.86 or 30 boards, assuming you by 16' ones.
If you assume you will use a combo of 6' and 8', you'd need 30 of each.
and if you want to go all 8' and cut down, it'd be 60 boards.

Existing plan recap:
8' - qty 63 plus spares.
6' - qty 30 plus spares.


If you go east/west, you can stagger by 4 feet, adding some look to the deck, but you will have to add some joists, plus move your "main joists" out to the edges, or build trimmer boxes along the entire edge to hold up the tails.  It looks like you are after the simplest method, so to do this one, move the posts.

84" / 5.625" = 15 boards running the length of 350" (just under 30 feet.) If you use 4' on center joists, with 8' planks, you'd use 4 boards per run or 60 boards, for the short deck to run the whole length across.
96" / 5.625" = 17.06 = rounds up to 18 boards, by less than 3/8ths of an inch, so I'd just mask it by gapping just a teeny bit wider on the last few boards.  Put in the end board and lay the other 3 or 4 down without mounting, take a tape and calculate out the gaps needed. The 14' feet width is a bit awkward so you'll use both 6' and 8' (18 of each flavor) and you'll have to add at least one extra joist, on one side, to continue the stagger neatly.  

East/West recap:
8' - qty 78 plus spares 
6' - qty 18 plus spares
Additional span joists:
6' - qty 12 
8' - qty 6


So, a lot more wood, but a lot more look too.