Hi iaak,
If your CAT scores are not improving, then you do NOT need more CATs, you need better/more practice materials. A CAT is a 'measuring device' (in the same way that a scale measures weight). It won't make you a better Test Taker, but assuming that you take the ENTIRE CAT under 'test-like' conditions, the CAT will give you a reasonable assessment of your score and point out your weaknesses. To improve, you need to study, work on practice problems, review CAT results, revisit past material, etc.
I'm curious about your studies so far:
1) How long have you been studying?
2) What materials have you used?
3) How have you been scoring on your CATs (including the Quant and Verbal Scaled Scores for each)?
4) Since you're asking about additional CATs, did you already use the 12 CATs that you bought already?
5) What is your goal score?
6) When are you planning to take the GMAT?
GMAT assassins aren't born, they're made,
Rich