56 mm = 5.6 cm since there are 10 mm in a cm, so we divide by 10 to go from mm to cm.
Since the diameter is 5.6 cm, this means 12 such cans lead to 12*5.6 = 67.2 cm of length needed to house 12 cans along this row.
Similarly, if we had 6 cans along each column, then 6*5.6 = 33.6 cm is needed for this
The diagram below shows what I mean. This diagram is a top-down view of the box. In other words, it's a birds-eye view.
The last dimension we need to take care of is the height of the box. That would be 2*5.6 = 11.2 cm since there are 2 cans along the height
Therefore, the smallest box possible has dimensions of 67.2 cm by 33.6 cm by 11.2 cm
You could have bigger boxes of course, though you'll need to place cushions inside so the cans don't rattle around. The smallest box dimensions listed above are ideal to avoid wasting too much cushion material (you'll still may need some form of cushion though just not as much).