You can set a minimum number of items for any product. If the minimum is 5 units, the customer will not be able to add less than that to their purchase. You can display the minimum on product page, and disable "Add to cart" for any amount less than the minimum, adding a special info message for the customer. An additional feature will disable from typing a figure below this minimum in My Cart section. Control the minimum amount from your Admin product page, and customize the info message in your theme settings.