I want to know if there is a chance of increasing height after 21. I'm going to be 21 soon and I'm 178 cm. I know for sure, the fact that my height is above average being an Indian, but I want to be 183 cm (6 ft). And if not, are those elevator shoes used by the celebrities worth using..

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.

You are at the age when the end caps of your bones calcify and at that point additional growth (at least in a vertical direction) ends. That can occur as early as your teens on average, normal healthy person.