Here is a simple module for Prestashop 1.6, which implements restriction on product quantity.

The standard Prestashop’s approach is to enforce minimal quantity, but to allow order anything above it. This module changes that behaviour in a way that customer can only purchase the quantity that is a multiplier of min quantity.

For example, if min. quantity is set to 3 – only following amounts are allowed:
3, 6, 9, 12, …

Download here: product_min_quantity