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 Or... "How many Tests should you create?" There are no "correct" or unique answers for these kind of questions. You should create as many as needed in order to ensure the quality of the product you're working on. The quantity and the timing depends on the process and methodology you have implemented. As generic rules, you should start testing as soon as possible and avoid testing only at the end of your development cycle. 

If you want to validate some components or subsets of the system, you may create specific Test Executions for it. The how and when depends on your approach.