I mean, it's hard to tell exactly how many users a single droplet can handle, considering droplets can vary wildly in size, and they may do everything (HTTP, DB, cache, etc.) or you might offload those other services to other PaaS offerings and only host the actual web code on the droplet.
That said, with a decently spec'd single droplet as a load balancer and a couple droplets as web servers and either DigitalOcean's database offering or another droplet running it yourself, you should be able to serve several thousand concurrent users without issue.
I'm making the assumption that your code is written relatively well and isn't doing insane calculations or anything else that is either disk- or CPU-bound.
Using Cloudflare as a CDN for static assets is a good idea and there isn't really anything you need to do there, other than put your servers closest to your main chunk of users. At the end of the day, data unnecessarily travelling the long way around the globe is going to make everyone have a bad day so if you can minimize that round trip for the majority of your users, they'll thank you.