It depends on the website and the amount of traffic it will be serving. A personal website that's not very popular will do fine on a 1-core Droplet with an email server, but for most of the time, it will be pushing the limits of RAM, especially if Swap is not configured.
It also depends on hwo you configure the email server. Any anti-virus feature will certainly drive up resource usage.
At my workplace, we're hosting dozens of personal websites on a one core server.
We also hosted one high traffic website on its own one core server. (the servers in questions were not on DO, but I am assuming the question is related to how much a cpu can handle)
If you put varnish cache in front of your website, you can handle hundreds of thousands of page views a day without any trouble(varnish is very good) and if your websites have no traffic(or less than 10 visitors a day) then you can host as many as your disk can fit without running into trouble.