It would require custom coding to limit one attribute among many others. If the "sample" size were offered all on their own, then it could be done with ZC out-of-the-box. But also I think would be difficult to manage as would have to update multiple entries if/when that product changed...

So, the controlled part is the addition of a product to the cart. An event is triggered that reads the directory extra_cart_actions. At this point, there is information being Posted and data that is already represented in the cart. Need to evaluate the items added specifically for the sample size (all other comparisons would be done by the routine cart actions) and process as setup by ZC admin. Ie. dump them with notification if the quantity in cart plus the quantity added exceeds the limit, or let them get added, but not able to process further (checkout), add enough to meet the limit, etc....

Basically would/could look at and treat all product with the option name and option value that represents 1ml (hopefully all use the same one and not multiple different representations) as a single product that is maxed at 30 or whatever number is to be the maximum.