There are many factors that contribute to the number of concurrent users that a website can handle. On a WordPress installation, a lot depends on what plugins you are using. A poorly coded plugin that leaks memory or hits the database too frequently can drag the site down.

The 8GB droplet should be able to handle quite a bit of traffic. Monitor the resources that you are using, and if you are seeing issues you can always scale up to a larger droplet. You might also consider setting up caching. Varnish is a very popular and flexible option.