There is not a way in Sphere to set a quantity limit by the attribute. An alternative would be to create a separate product for each selectable attribute as you can set quantity limits on the actual products for sale.

You can suggest quantity limits be included for attributes in Ecommerce for future releases of Sphere through our Idea Bank which can be accessed through the following link: https://www.blackbaud.com/nonprofit-resources/idea-banks.aspx This is where customers can go to suggest and vote on future functionality. The products team looks at the Idea Bank to see which requests are most popular to see what will be developed for future releases of Sphere.