How many groups of 6-12 numbers are there?
There’s some information about that on the how to memorize numbers page.
Edit: I just realized that you might be talking about the Shaper System when you wrote “I tried to associate a shape to each number”. Is that the system you’re using?
If you want to practice rapidly memorizing numbers under pressure (compete live) check out Memory League. You could recall them backwards and it should still work.