I think there are only 3 total. I took the 1st and 2nd tests, roughly 10 days apart. I was much better prepared for the 2nd than 1st test and my score only increased by 2 points. Based on that data, I would say the 2nd test is harder than the 1st (though others have said the opposite). In theory, though, I think all tests are designed to be equally difficult. Any difference in difficulty should be reflected in the scoring anyway!