How much concrete or postmix do I need per post?

That depends on the size of the hole. I would recommend keeping the hole as tight as possible. Digging a great bomb crater will require more filling. It is best to use 1 bag of postmix for each post up to 1.2m high out of the ground and 1.5 bags for posts longer than 1.2m. Postmix in 20kg bags should fill a hole of 300x300x300mm.If mixing your own concrete I would recommend using 25kg per post up to 1.2m and 40kg for anything longer to ensure it remains stable.