According to Envato Elements' licensing terms, a single license can be used for a single end product. In this case, each subscriber's portfolio website would be considered a separate end product, so a separate license would be required for each one.
To be sure, it's best to contact Envato's support team directly for clarification on their licensing terms. You can do this by visiting their support page and submitting a ticket.
// Example code for submitting a support ticket on Envato's website
// Replace "YOUR_MESSAGE_HERE" with your specific question or concern
// Navigate to Envato's support page
https://help.elements.envato.com/hc/en-us
// Click "Submit a request" in the top right corner
// Fill out the form with your information and "YOUR_MESSAGE_HERE"
// Click "Submit" to send your message to Envato's support team