How many Flexcubes can be safely stacked on top of each other when full?

Up to 4,000L/40 hl/1,060gal worth. That is equivalent to: 4×1,000 litre Flexcubes or 2×2,000 litre Flexcubes or 2×1,500 litre Flexcubes + 1×1,000 litre Flexcube.