2.592 cubic metre. If you are having it delivered in builders bags you will need three. 1 cubic metre is 1000 litres, so 2,592 litre.if 60L compost bags is 43. 40L compost bags is 65. That is hell of a big planter. What are you planning on growing in it?
You need a soil based medium, not compost on it's own. It won't have enough sustenance for plants that are in there long term. You could probably put a layer of rubble or similar in the bottom to save money, as it's a deep planter. If you had some spare turf, that works very well. I did that with all my raised beds as I was removing an area of compacted turf from the boundary of the garden. It's ideal for long term planting.