Brick sizes and joint widths differ by country and by product, so a fixed number per square metre is always wrong somewhere. Enter your actual unit size and joint and this works it out from geometry, then adds the mortar.
Notes
Where the count comes from
Each brick occupies its own size plus one joint in each direction. So the area of one unit including its joints is (length + joint) times (height + joint), and the count per square metre is one divided by that. Nothing is assumed about your brick.
Mortar volume
The mortar is the difference between the wall volume and the volume of the units in it. Add 10 to 20 percent for what falls off the board, stays in the mixer and fills frogs and perforations.
Single skin, double skin
A wall two units thick uses twice the bricks and more than twice the mortar, because the collar joint between the skins is mortar too. Set the number of skins and it is included.
What this does not do
It does not specify a mortar mix, allow for wall ties, movement joints or reinforcement, and it does not know whether your wall needs a designer. It counts material.