How many bricks do I need?

About 6.9 for standard US modular brick laid in a running bond with a ⅜ inch joint. The figure changes with brick size and joint thickness, which is why both are inputs here rather than assumptions — a queen or utility brick covers more wall per unit and the count drops accordingly.

Ten percent is the usual allowance for cuts, breakage and the odd rejected brick. Add more for a wall with many openings, corners or a pattern bond, since each one generates cuts. Order it all at once: brick colour varies between firings, and a later top-up can be visibly different.

No — this counts bricks. As a rough guide a standard bag of mortar lays somewhere around 100–125 modular bricks depending on joint size and the mix, but take the coverage from the product you are actually using and add sand separately if you are mixing on site.

Yes. Enter the net wall area with openings already taken out. For a wall with a lot of openings it is worth adding a little back, because the cuts around each one produce offcuts you cannot use elsewhere.