Either solution could work. If I was to do this on a single server I would likely have multiple discourse containers with a single copy of nginx using proxy_pass to assign each discourse instance to a different subdomain/domain or directory.

This setup should be lighter on resource usage than having a single droplet for each instance but knowing just how much lighter would mean some testing. It would, however also mean that you’d have a single point of failure for more of your forums and could be a bit more complex to administrate.

On the other hand you could also choose to have a number of droplets each running one or two discourse forums with your nginx reverse proxy running on a droplet of it’s own, passing public requests across the private network to your discourse droplets.

I would recommend asking about the options you are considering on the discourse meta forum as there are likely people there who have real world experience with different methods and they might have more input on which methods would better meet your needs, how many discourse containers you can run on a given amount of resources and what worked best for them.