Qantity Product module

[Lucie]

Hi

Before i start i'm sorry for my english...

I'm trying to create a module for my website who can modify the quantity product adding to cart. 

Let me explain, when you are on the  product file as a client you can order a product one by one.  I want to order a product one by one till 10 (like 1,2,3,4,5,6,7,8,9,10) and when you reach 10 it's goes 10 by 10 (like 1,2,3,4...10,20,30,40...). So i want to modify the quantity product to order (next add to cart)

I don't know how to precede...

Any help ? 

Edit : I'm on prestashop 1.7 on local

Edited September 29, 2018 by Lucie (see edit history)