It is almost impossible for anyone to tell you how much resources you would need, as it is really application specific and it also depends on how much traffic you would be getting and etc. but of course I could suggest the following approach:

• As you would be paying by the hour and not on monthly basis, you could start with a smaller droplet and scale up whenever needed. You don’t have to commit to a specific plan, it is really flexible and you can resize your droplets, add new ones at any time and you would only be charged by the hours.

• Also once you have your setup ready, I would recommend running some stress tests to see how many connections your setup can handle and then you will be able to predict wether you need to add more resources or not depending on the expected concurrent connections that you need.

• As you would have a database, I would suggest using the new DigitalOcean managed databases, all of the management part would be handled by DigitalOcean and you can focus on your application.

• With the managed databases you can also have multiple read-only replicas with just a few clicks, so that way you could scale up your application even easier.

Conclusion:

Just sign up for a small droplet, run some stress tests and scale up accordingly.

Here’s how you could run stress tests:

https://www.digitalocean.com/community/tutorials/an-introduction-to-load-testing

Hope that his helps you :)