It all depends on the quantity and weight of plants you will plant on the ground. Generally, to fill a 3×6 bed with 10″ sides, you need 15 cubic feet of blended soil. When you create the blend, you can use 9 cubic feet of topsoil and 4.5 cubic feet of compost to make the perfect soil for your plants. You can also go through different soil types on Shops.ae which has briefed each product description. It will get you an idea of how much soil you will need for your garden.