A Group is a set of test cases that use the same PHYs. If you have several test cases in a test session, and all the test cases use separate PHYs, then there would be one test case in each Group, that is, Number of groups = Number of Test Cases.
However, if some test cases share PHYs, the TCs that share PHYs would be together in the same Group. So in this illustration, Number of groups < Number of Test Cases.