The 15 Veritas tests include tests that are only available at Veritas -- Veritas Prep Simulator Tests and GMAT Life Tests as well as other tests including the GMATPrep.
Most of these tests cannot be retaken. I always thought that 15 sounded like plenty...
You asked my opinion of whether the Veritas tests were similar to the GMAT. Like other company tests they are similar in many ways, but are not exactly what is on test day. For example, the math on the Veritas Prep Simulator is more difficult and varied than on the actual GMAT test and it scores harder as well.
By the way - in addition to the 15 CATS Veritas students also get diagnostic quizzes in various areas - for example critical reasoning, sentence correction, arithmetic, algebra and these quizzes are very helpful in diagnosing areas that might need improvement - as well as areas of strength.
As far as CATs go, Brian Galvin discussed in his article this week "my practice test, my rate problem" with practice tests you really need to focus on whether you are doing things right - especially with not official practice tests - rather than focusing only on the score.