The set comprises of 20 levels with each level having one CD. The idea is to start from level one and progress upwards. It is recommended that five levels are completed with the rest being optional depending on your own personal development needs.
I will report more about when I know more. Looking at the way these are produced I think they will probably place a limit on the number of CD's which they can release for sale. More information can be found on the publisher website. It looks like it is still in the process of being updated but I am told will be completed in a few days.