Yes, you can host multiple sites on one server (Droplet). Naturally, the number of sites will depend on traffic and web server technologies. There is no absolute answer defining the max number of sites, since it depends on so many variables.

You can install any e-mail service on a Droplet. Furthermore, many CMS solutions provide e-mail-based user registration.

Sure, you can host multiple websites in VPS
this article explain how to host multiple sites in Ubuntu-apache2

follow “Create on Home directory” topic