You can start out with a single droplet (server) and use a reverse proxy like NGINX or HAProxy to route traffic based on domain or sub domains to your applications.

I used the How To Set Up a Node.js Application for Production on CentOS 7 configuration for my Node.js applications a few times, and I recently helped to update the tutorial (because I think its a solid foundation).

I have a few sites in production (one is a Next.js app) that are essentially following the same setup. You could use this configuration to host multiple sites/Node.js applications, just by adding extra Node.js processes to PM2 and then the Nginx config.