There is a chance you may become taller after 21. Its commonly thought that 21 is the age most men stop growing but it doesn't mean you can't.
How much you may grow is anyone's guess. There is no way to guarantee you will grow a certain amount. If you're insecure about your height, you could invest in elevator shoes but they would be noticeable to those around you. I'm not sure if that matters to you or not.