You can determine the quantity of items that can be ordered. For eg., items can be

ordered in the multiples of 4. (4, 8, 12 etc)

You need to write the following rule. (Items -> New/Edit -> Next -> Rule (valid PHP-

Scriptcode, double quotes are not permitted!) for this item. Check FAQ
Where to enter Item Rules?

$units = 4;

if(article_get_field('count') % $units != 0) {

article_set_field('HIDE_ORDERBUTTON', TRUE);

article_set_field('hint','Please choose a multiple of '.$units.' as the quantity');

} else {

article_set_field('HIDE_ORDERBUTTON', FALSE);

article_set_field('hint','');

};

Please note:

A customer can only order items in multiples of 4. The order button during the final checkout (along with other items in the basket) will disappear if the quantity is not in the multiples of 4 and a message is displayed.