One extension license is per one Magento installation. It means that if you run your multiple stores by the same Magento installation (Magento allows that) you don't have to buy additional licenses. If you are planning to use our products within several different Magento installations, you got to buy as many extension licenses, as many Magento installations you have.
However, there's one more thing you should consider: if you purchase our products, you may install them to your Magento development website. We assume a website to be development if it's not public and if it has no real orders / customers. In this case you don't need to have additional licenses. You may install extensions to your production and to your development websites under the same license.