I want to host hundreds of instances of Discourse

I don’t recommend going multisite with < 4GB of ram, but Multisite configuration with Docker.

From what I can tell about your skills and needs, I’d recommend going with 1GB droplets for each city until you have at least 5. In the mean time you can experiment with multisite on droplets that you spin up to test and then delete.

I recommend that you wait until you have a problem before you go about solving it.