Go to a product variant details page and set 'Minimum cart quantity' property
oooh the big guy is here ...
hi Andrei Mazoulnitsyn ,thank you fro your replay.
but what really need is to set a specific number for the total quantity of all items during the check out step. please take a look to this image <img src="http://jhpdxa.blu.livefilestore.com/y1pcDytXLf8muZr1ejFnF8k6TxiMMpMk_OCjktHQt7Y4GWRi4gQzhNwqfXp-xpuMYV7yhim8UVtTuhG0VotvEeVqMbFqn_B35UW/foto.png?psid=1" />
as you can see i want to calculate the total quantity for all items not for a single item and make a validation on the total (if the total quantity is more less than 24 then show message to the user to inform him that minimum order is 24.
for now I added required check-box to solve this one temporarily until i publish the website.