It Is Quite Simple How To Work Out The Number Of Bricks You Will Require, But There Are a Few Considerations You Must Consider

1- Thickness of the wall to be built

2- Amount of openings
3- Amount of detail in the wall ie: Decorative panels, Dentilation, or Dogtooth String Courses
Knowing the thickness of the wall is the most important thing to know.
The rule of thumb for working out the number of bricks, you will have to work out the total area of your building you are going to clad with brickwork, for every square m2 you have,

you have to multiply this by 60. So if you have a wall that is 6 meters long and 2 meters high, you will have a square area of 12 m. So we will need to order 720 bricks.
Please allow about 10% wastage, this can be due to transport or just moving the bricks around your project, some bricks can be quite fragile: so to clarify:
Single Skin Wall
→ 6m x 2m = 12 square metres x 60 (number of bricks in a square metre) = 720 + 10% wastage = 792 bricks required
Double Skin Wall
→ 6m x 2m = 12metres x 120 = 1440+ 20% wastage = 1728 bricks
Note: On the double skin wall I would allow for a little less wastage.