The GMAT Prep software from the GMAC (Graduate Management Admission Council) uses a
pool of questions, not an unlimited number of unique questions. While the exact number isn't public knowledge, it's a large pool, and the algorithm tries to ensure you don't see the same question twice in a single practice test. However, if you take many practice tests over a long period, you *might* eventually encounter some repetition. The questions themselves are also frequently updated.
So, the answer is nuanced: It doesn't use completely different questions *every* time, but it strives for significant variation to provide a realistic and challenging GMAT experience.