There are, indeed, a large number of these exams.
When you log in to your veritas account, there is a box with a blue header at the bottom right on the "Practice Tests" tab. Here, you will be able to input when you'd like to take the actual GMAT, and the number of hours per week you intend to devote to GMAT study during the course. It will then generate a schedule for you that includes homework as well as practice exams.
Generally, though, in an ideal world, I like to see my ambitious students taking a practice exam as frequently as every week to ten days. This works on topics already covered in class as well as more intangibles like the ability to handle problems on the screen instead of paper, timing of the overall test, and test-taking stamina.
None of these exams are designed to work on a specific topic within the GMAT, but rather, are GMAT simulators designed to look and act like the real test, including all topics and question types.
Hope this is helpful,