I saw this in "Uber" there is a progress bar that shows you how many referrals you have signed up, and the bar has all the milestones marked on it.
It shows them how much more they have left before they unlock the next prize, all the way to the max prize given.
Physiologically makes them want to "complete" or fill in the bar while sort of taunting and encouraging them with how close they are to the next milestone.
"its a cunning little piece of gamification — everybody wants to fill up progress bars"
Adding more templates which includes a progress bar :)