A conversion or usability consultant can suggest hundreds of changes to a single screen. Testing will let you know which of these changes will work. Testing a few changes on a page is a good starting point; testing thousands of changes can be overwhelming. In most of our projects, we limit the number of scenarios we test to fewer than 100. Of course, in some instances we’ve tested more variations of a single page, but these are the exception. You should also consider the limitations of the testing software you use, and how many conversions it requires for the test to conclude in a four-week period. As a rough estimate, Google Website Optimizer requires 100 conversions for each scenario you test.