If you are growing vegetables in your raised garden bed, it is worth using a premium grade of soil, not potting mix. Potting mix is designed to hold a lot of water, but for a raised garden bed, it is better to have a well draining soil.
For example, a good soil that we like to use when making our garden beds will have ⅓ rough sand, ⅓ clay, ⅓ peat/sustainably sourced shredded coconut husk. Depending on the size of your raised garden bed, it may be more economical to source this directly from a landscape supply store rather than buying bags of soil from a supplier like Home Depot or Bunnings.
For the Subpod Grow Bed on our website which is 90cm/ 3 feet long x 120 cm wide x 45 high; after subtracting the volume occupied by the Subpod in the garden bed, you will need approximately 0.35 cubic meters of soil/340 litres of soil, e.g. 7 x 50 litre soil bags or 14 x 25 litre soil bags.
For a larger bed, say 90cm/ 3 feet wide by 1.5 m/ 5 feet long, by 35 cm/ 1 foot high; after subtracting the Subpod volume, this requires ¾ cubic meter or 1 cubic yard of soil, which is 750 - 1,000 litres of soil, e.g. 15 x 50 litre bags of soil or 30 x 25 litre bags of soil.
I would add a maximum of 20% compost to the total.