How many Players/Users/Slots can I have?

I have a VPS with 4GB RAM, running CentOS, with Essentials and Towny. How many slots can I have for this server?

 

I'd estimate about 1-90. It all depends really on how well you optimize to handle the players.

 

Where can I find some ways to optimize?

 

Practice good plugin management. (Only use plugins that are up to date)
Limit resources. (I never use a server render distance over 3, that's just me. You can also limit things such as world entities and such with plugins like Worldguard or Nolagg)
Allocate more ram. (I'd say at least 4 gigs is all you need. Can't hurt to add more though.)
Use a fast internet connection. (Ensuring that your server is running on at least a 1G port connection would be great.)

As far as optimizing your machine? There isn't much to that, just make sure you keep Java updated. Dedicate your memory toward your server process. (I'm not sure on how to do that with Cent) Can't really think of anything else, try a quick "How to optimize a minecraft server" Google search to find more.

 

OpenVZ? You'll be lucky if MC will open... And stay running... And not crash... Or get killed for abusing swap...

Xen or KVM? 40+, assuming a decent but not heavy load of plugins and sufficient CPU resources.

 

You can have as many slots as you want, but I'd recommend 40-60 assuming RAM is your limiting factor.

 

For me I find that the limiting factor is usually the CPU, and if you have bad plugins. Just test it out and keep increasing if you have that many people on.

 

I usually have between 30-50 players with only 2gb and 30 plugins.
As long as you get ones that are not too heavy and badly optimized you could handle a good amount with 4gb.

I had started off with 40 plugins but we were small, as it grew I noticed what were the problems and removed them.

 

http://canihostaminecraftserver.com/

 

You could as well ask your grandma to estimate the possible number of slots on a minecraft server.