I remember a similar line " How many people does it take to change a light bulb"

Q.How many psychiatristis does it take to change a light bulb?

A. None–the light bulb will change when it's ready. [courtesy: wikipedia]

                                           OR

Q. How many minions does it take to change a light bulb?

A. https://www.youtube.com/watch?v=17_VuGuQJVs [courtesy: youtube]

Coming back to your question, if it is change the world to a better place, then the answer will be approximately 7.2 billion (which is the world's population). Only if each of one of us can change, this would collectively be possible. But, since you mentioned that this is a trick question. let us know the answer to this.