How to setup a group sale like - if more than 50 people buy this product you will get 20% discount?

[uriab]

All existing group sale apps I found were asking users to invite their friends and the deal sale counter was counting the number of friends invited per user.

I want to setup a global counter that will count ALL sales of the discounted product and if sales go above 50 then all people who bought it will get a 20% discount.

Please help.