Free-standing deck joist layout
I would suggest using software to determine a reasonable design. Lowe's has an online deck designer. There are lots of others, too, like Design-a-Deck. 

The most obvious omission from the design is that there are no beams. Where is the house?

Although railing posts can theoretically be slapped on anywhere, the layout of the posts will change its aesthetics a lot. Decks with the usual slapped on railing posts tends to have that chock-a-block treehouse look to them.

The structural design can vary a lot depending on how solid you want the deck to be, how much creakiness you can stand. A completely silent deck will be a LOT more work and money than a slightly creaky deck.

I notice that very high quality decks tend to have trusses or even cross bracing.