You could offer additional slots on the basis that very few servers will actually use a lot of resources.
Yes, that is true, but I like to keep a good sized reserve available for when the servers hit peak times. Currently it's running about 55% of the total capabilities with 18 servers and few players on the servers, but when I hosted that crazy Steam party server with 99 players the day Blockland came out on Steam, plus about 45 other players spread out on other servers, I was at about 85-90% with 18 servers running. So I'd say 20 total is about right for the node we have now, and that takes care of any big player counts we may have.
I may increase the limit depending on what I see over the next week as far as traffic and resource usage goes, now that RTB is going down and people are going to move to the servers that are still up. If I do increase it, I'll let people know.