How many sessions are you looking to run on each server, and how many servers?
I've got experience of running upwards of 600 Sun Rays (with a mixture of pseudo (non card) and card sessions so the number of sessions will be considerably higher than 600) per server.
In a recent implementation we put in around 3500 Sun Rays plus iPads/OVDC connections spread across 6 dedicated physical servers; though this was on Solaris I'd expect similar on Linux.
Would be good to understand a bit more about your specific environment, what type of session(s) you're using, and what kind of numbers/resources you have available.
If it's a purely an academic question I apologise for the above noise