Can I back up more than one site?

Yes. Each plan covers one site address. Start a plan per site with the same email and they all show in one dashboard.

A plan is tied to one site address, because that is what gets crawled every month. For a second site, open your dashboard and use "Add another site", or go to the pricing section and use the same email. Every site under that email is listed in the same dashboard, each with its own backups, its own monthly email, and its own cancel button.

Designers who look after client sites: the designer page gives you a code that takes half off each client's first year, and your own site backed up free. Pay for a client's site yourself and put their address in "send the monthly backup to", or hand them the code and let them pay.