You are asking very subjective questions.. The tests in Kaplan
CD are tough and the scores you get would be about 100 less than what you could get on the actual test. Quants sections are quite good. Verbal sections are very difficult.
Keep 2 things in mind. Don't take Kaplan
exams if there are not many days left for the actual exam. And however tough or irrelevant (sometimes) the verbal section may seem try to concentrate hard and get the questions right.
Another option may be the tests in Princeton CD that comes with the book. The scores from Princeton test will be close to the real one though the questions may not be.
About optimal number of tests, there are not many good tests available. Apart from the 10 tests (ETS 2, Kaplan
4, Princeton 4), you will rarely get consistent(whether good or bad) feedback about tests. So even if you want to take you may not get many that are really useful.