I typically divide the wall height by 6.5 (which is the face measurement of the board, the area you see...) the remaining 3/4” is the “lap” part that is tucked behind each board. 92/6.5 comes to 14 .15 boards. You can always return a board or two that you don’t use. But it’s better to have an extra one just in case you experience a mis-cut around an outlet or something. Best to you and your project!
The underlying edge is longer than the overlapping edge. Tap in till it is tight and there you have it. I did use a level to make sure I was staying in line.
I used two 2-inch brads in each board, one in the tongue (hidden) and one in the face which you need to fill and paint.
Per the manufacturer's spec sheet ... "It is an interior product, not to be used in outside applications". So, I guess you do so at your own risk if you choose to use it for an outside application.
Since it's real wood, bathroom installation shouldn't be a problem. I'd put a rough coat of paint on the inside surface, lust to be sure.
The grooved side is to prevent warping and is not as smooth as the face side, but, I suppose, if that's the look you are going for, you could do it.
We've used this product in several ceiling applications, both the six and eight-inch versions and we have found that either nailing pattern works well. However, if you nail into the tongue and place one nail on the face, you will have fewer nail holes to fill. We used two-inch, 18-ga. brads, but finishing nails or 1/4-in. staples will also work but leave larger holes. We use matching 1 x 2s around the edges and paint before installation. They look great, consistent gap, straight boards.