If anyone is interested in stress-testing, hop on Crown's Jailbreak, it's now open and is being hosted by Glass Hosting.
Most systems are in a workable state and I'm fairly happy with them. I still need some improvements with user-friendliness and none of the site pages have any design on them still, but we're still on track.
You should use this: https://github.com/portify/BlocklandLoader
I don't know if it works on linux servers though.
Yep, that's what I'm using and it seems to work fine. Some minor issues with file discovery, but that's more of a wine/linux issue than anything.
I think you'll be able to get 20 players in a server, albeit I have no information to back this up.
Just for some comparison, I believe Crown's server is a pretty good benchmark as it's often the most popular and has a fairly large amount of stuff going on. His server idles around 20% CPU when empty (which is 40% of his share) and I believe around 30% of his allotted RAM. Right now, with 20 players online, CPU usage is only up to 25.1% (50.2% of his allotted amount) and is using 48.8% of his allotted RAM. Following that math, he has enough CPU to have ~120 players and enough RAM to have ~75 players.
edit: I doubt anyone will ever need this, but fully private nodes will be available on request, allowing those estimates to be doubled, which is beyond Blockland's capacity afaik.