Hi,
You don’t need min & step here, you can make a good use of “Allowed Quantities” under Fixed quantities tab.
Enable it, tick “per product”, and then on the variable product edit page, you will see a new field by the plugin called “Exact quantity allowed for all variations”, enter 6,12,18,24, etc… for the total quantity you want to allow for this variable.
Please let me know if this doesn’t solve it.
Omar