It is natural that we should want to put as much onto the shelf as we can to maximise the number of pieces from each firing. But, when you are placing the pieces remember that glass expands as it heats up. When the glass is at its maximum expansion, it will be much less viscous than at lower temperatures and so will stick very easily to any neighbouring piece it touches.
Although the final size of a two-layer piece is the same at the end as the beginning, they do expand to a larger size during the fusing process. My experience shows me that a 6mm piece can expand as much as 5mm, depending both on temperature and size. This means that I treat 10mm as the absolute minimum space between pieces. But, because of the size of my fingers, my normal minimum placing is 20 mm apart as that is a comfortable space between my fingers and the other glass.
Thicker pieces expand to become larger after fusing than they were at the start. These pieces spread more during the firing than the 6mm piece. A 9mm piece may expand by about 3mm at the finish – depending on size and temperature. But during the firing, it may expand as much as 9mm. This means that 20mm is an absolute minimum between pieces that are 9mm thick at the edges, even though they may be only 6mm over most of the area.
The tip is to avoid over-filling your kiln shelf. By trying to get too much production in one firing you may find a number of pieces stuck together at the end, thus eliminating any savings on glass or space.