Test Scenario are the possible conditions for which the functionality in the application will behave , we need to track all possible behavior of that particular functionality ,it may positive and may be negative.
Based on the Test Scenarios for a requirement we will have to generate the Test cases.
Suppose we have 10 Test Scenarios for requirement then we have cover all those 10 test scenarios in one or two or three (one to many mean one test case cover all ten test scenario) test cases.