I tested it a bit and had 2-4 seconds with 100 guys online (depends on click activity though).
Looking at the console the CPU was busy about 100% most of the time, going twice in 15 seconds to 90% busy and then up again.
I can make it higher, but it will run slower. And even 10 guys do make a difference (compare on the old server 60 -> 80 ... made more than a few seconds).
With a second server, I can push this one normally up to 150 (that's how it runs on another machine I know of with exactly the same config then: 2 servers). But with this, I feel 100 is ok.
Yet as donations still keep up, I save money for a second server. That should take some load off and then I can increase the limit. I can't say when. =) as the monthly costs will rise then.