Try Manhattan GMAT (MGMAT)

You get 6 free full-length tests online for purchase of any of its prep guides.

My personal opinion (Compared to GMAT Prep):

Question Quality: About as good as GMAT Prep
Question Difficulty: Quant --- seems harder
Verbal --- approximately the same
Test Scores: All Quant, Verbal and Total scores
---- seem higher than GMAT Prep.
So don't be too happy if it barely hit your goal, expect a drop in the real test.

Despite of the inaccurate score prediction, both the question quality and difficulty shall give you great training before the real test.

You may spread it out, probably 1 MGMAT mock test per week. Then save your last GMATPrep mock as your last test.

Good luck

Actually, you shouldn't aim to take "as many tests as possible." Focus on taking tests that are representative of the GMAT, and take the time to fully analyze your results. The quality of your materials and your strategy is far more important than quantity. Quantity is immaterial if you're doing it right.

Create an error log so you can find the patterns in your errors. Review the answer explanations and make sure you understand them. This forum is a great place to ask questions.

Other than GMATPrep, I feel Princeton Review's tests are the most accurate.