How much Stain should I buy?!

Last fall I had my old deck resurfaced with PTP and expanded. It's now 17x22 deck in need of cleaning, brightening, and then staining. None of the products I want are sold locally so I will have to buy online and most likely in 5 gallon quantities. I'm asking all the folks that stain decks regularly if 5 gallons will be sufficient. It will probably need 2 coats as this will be the first staining. It has standard railing with balusters and wooden lattice around the bottom.

Any input would be appreciated.. Thanks!