How to create minimum amount of items to purchase?

Dear forum members,

I’m trying to create a request for two particular products: minimum amount to purchase 50 items with a step value 1.
I’m using Woocommerce for the shop. And it’s pretty tricky for me as I’m scared to ruin completely the code.
Any idea? I want to apply this rule for this product: http://olivia.be/product/encart-couleur/?lang=fr

Thank you very much in advance!