Still figuring out the numbers, so very hard to answer at the moment. Chatto is quite lightweight though; a fresh instance will run at tens of MB of RAM usage. RAM usage then scales with activity and CCU; on the Chatto HQ server, I'm currently seeing around 10 MB per additional connected user. I want this to be less, there's probably lots of room for optimization.

(You can scale Chatto horizontally across as many processes and servers as you need, so communities with tens of thousands of CCU should be perfectly feasible. But as you can imagine, there's precious little real-world proof for this yet :b)