by Jim » Thu Feb 23, 2012 2:43 pm
There is no built in way to limit the quantity of a product that can be ordered. (Pro level stores can impose a minimum number that must be purchased.)
To work around the no maximum limit you could create some javascript that would check the quantity of an item in the cart and if it exceeds the limit not allow the shopper to checkout.
Some merchants have created forms on their pages with specific quantities that can be ordered and then turn on the "Quantity can not be changed" checkbox on the Commerce Setup > Order System > Shopping cart screen. This makes it so the quantity can't be changed in the cart which might prevent some people from trying to purchase additional items. However, they could go back to the page and add the item to the cart again to get around the limit. So even doing this you would probably want to add the javascript check and prevent them from checking out with more than the limit.
Check this post that give some suggestions and a link to Lexiconn's tutorial on the subject.
http://support.shopsite.com/forums/viewtopic.php?f=1&t=8814&p=23028&hilit=maximum+quantity#p23028
Lexiconn link is at http://support.lexiconn.com/news/viewtopic.php?t=415