@JasonT - There’s not a finite limit on the number of domains an individual droplet may host. As each site is unique, and as each site will surely have varying traffic levels and resource needs (i.e. CPU, RAM/Memory etc), the true limitation is resources.
A domain, on its own, doesn’t require anything more than the disk space consumed by the lines of the configuration files written to tie it to whichever web server you choose to use (i.e. Apache, NGINX, Caddy etc) until it begins receiving traffic.
That said, 512MB is enough to handle a few low-traffic dynamic websites without an issue. Of course, it’s hard to put a finite number out there as one site may be more active than another, just as one site may tax certain areas of the server more than another (i.e. a WordPress site, which requires PHP + MySQL is going to use more resources than a static site consisting of just HTML & CSS with a few JS files, such as jQuery).
To be more direct, though, you could technically host “unlimited” domains as DigitalOcean doesn’t limit you by the number of domains you may host. Since Droplets are “bare-bones” (i.e. you’re responsible for administering the droplet and should be fairly comfortable using the CLI to do so), you can host 5, 10, 50, 100+ - in the end, resources will be taxed and maxed out well before you hit any undefined arbitrary limit on domains :).