I have a Wordpress Website (techorfy,com with Wordpress+Nginx) on a Digitalocean 5$ plan. The busiest time is like 8 Visitor per seconds and page load time is like 2-3 seconds.
Currently, the RAM usage is 70% and about 155 MB is free (on 3-4 Real time visitor), so is the 155 MB free RAM is enough to handle traffic spike like 100 real time visitor.