The discussion revolves around the minimum server specs for hosting a multisite Discourse. tenzan initially assumes that doubling the RAM from 1GB to 2GB would be sufficient to host two domains, but codinghorror recommends a more conservative approach: at least 1GB RAM for the first community and 512MB RAM for each additional community. tenzan then asks about swap size and Sander78 quotes a Linux guideline that recommends swap size to be at least equal to the RAM size, although this can vary depending on the applications running. tenzan notes that their personal experience suggests a swap size of twice the RAM size, but seeks Discourse-specific requirements.
Recommended environment:
Dual core CPU recommended
1 GB RAM minimum (with swap)
64 bit Linux compatible with Docker
The above is for 1 domain, I assume.
What are the minimum specs requirements for multisite discourse?
If I want to host 2 domains, does it mean I will need at least 2GB RAM ?
How about hosting 3 domains?
I found this Do I need a VPS for Discourse and do I host the main site on the same server? - #5 by Sander78 and it seems I can host 2-3 discourse sites on 1GB RAM droplet.
Please correct me if I’m mistaken.
I would not recommend that, no. I’d say 512mb ram minimum for each additional community.
So 1 GB to start with community 1, then + 512mb for community 2, + 512mb for community 3, etc.
Thanks @codinghorror 
Note that I said I have very small communities. And I mean very small. So you might want to go with @codinghorror’s advice.
Let’s say, if I use 2GB droplet, for 2-3 sites, do you recommend to have 2GB swap as well?
If I’m not mistaken the recommended size for swap is always the RAM size. But hey, Google is my friend and I found this:
http://wiki.linuxquestions.org/wiki/Partitioning_guidelines
From the Swap space paragraph:
Almost every Linux system will need swap space. The classic piece of advice is “swap should be double the amount of RAM you have”, though this is not a rigid guideline. Another guideline is to have at least 1x your RAM size, but this also isn’t a fixed, absolute requirement (this used to be necessary for linux, but not anymore for recent versions). It depends mainly on what applications you want to run. Applications like video/movie editing, 3d rendering, large image editing, database servers and web application servers can use a lot of memory and if you don’t have that as RAM, it will need a large swap space.
I was wondering if there is some Discourse-specific requirements.
Otherwise, from my experience, it was always recommended swap size = RAM x 2
At least from my experience.