The GMATPrep CATs are the best, so definitely take each twice. Make sure to space out when you take them so that you are somewhat fresh for each retaking (more on that later). The GMATPrep Question pack is excellent as well and you should buy those retired questions. That said, the QPack is a different beast, as you can use it for practice to improve your timing and target weaker areas.
Depending on how long you have to prep, you very likely do not need to take more than 10 total CATs. I like the MGMAT CATs - I used them during my prep and they have a great reputation. Veritas is a well established company and I'm sure their CATs are fine too. It is possible that their Quant sections are a bit on the
easy side which contrasts with MGMAT being a bit tougher in Quant.
Either way, it probably makes sense to use all MGMAT (because they are proven) or use half MGMAT and half Veritas (to balance out eccentricities in the tests). Use the GMATPrep CATs as the measurement of your overall progress and the 3rd party CATs for honing skills, practice with timing, and building mental endurance.
Here is what your full CAT schedule could look like with the six MGMAT CATs mixed in:
GMATPrep 1
MGMAT 1
MGMAT 2
GMATPrep 2
MGMAT 3
MGMAT 4
GMATPrep 1 (retake)
MGMAT 5
MGMAT 6
GMATPrep 2 (retake)
Here, the retakes are far away from the first taking, which makes the GMATPrep retake scores more reliable as an indicator of your current score level.
It is important to note that most improvement on the GMAT comes from learning/reviewing strategies and taking practice problems on specific topics, not from taking tons of CATs. CATs are crucial for getting used to the official experience and other things, but be sure you strike a nice balance in your prep.
Incidentally, I've just published
GMAT Clarity, a guide to GMAT self-study. It answers questions like this very thoroughly. It's currently only $7 on Amazon (will soon be $40).
