You get two practice exams with the challenge purchase, which should be enough. You can purchase more for $100 each (I believe), but starting with the second one, you start to see questions being reused. You’re going to have diminishing value with the third, fourth, etc. They might let you purchase just a practice exam in advance if you’re that nervous about taking the challenge.
I took GPEN test at the end of February, and felt that the test exam is a bit more difficult than the practice test. If you are going to prepare yourself with the practice test, you could do it once. I did it twice, and found that the questions were reused as ajohnson said.
Make sure that you prepare with the contents of the materials. You will be fine.