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? 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.
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.