How many slate tiles you get per square metre really does depend on the size of the tiles. Establish the width and length of the tiles in mm, multiply these together, this will give you the area of the tile, then divide 1 x this area and this will give you the amount per sqm. Example below:
600mm x 400mm size tiles (0.6 x 0.4 = 0.24). 1 / 0.24 = 4.166 giving you just over 4 tiles per sqm in this size