7-10 fighters I think all in the same system shooting at each other. I didn't get a chance to see how many people were on the server, but it was a lot.
The lag was manageable, 2 or 3 times we had yellow or red. I experienced 2 'hiccups.' After an extended fight the server crashed or went off line with no warning, not sure.
I caution everyone not to judge the performance of the server purely on what the gameplay is like for those of us with top-rate systems. My system is showing it's age these days so I'd appreciate some conservativism when looking at how many slots to open.