How many simultanious users

[Lazy]

Hi,

I'm trying to figure out how many simultaneous users a single 8 core
machine with local apache running as a backend can handle assumming
that all the requests are cached.

testing with ab on a slow 100Mbps link shows 2500 hit/s, locally i got
12 000 hit/s with over 200Mbps traffic

assuming that each user loads 40 files in 1 minute we get
12000*60/40=18 000 users per minute

Is it possible to get half of that 18k users/per minute in real word
ignoring the amounts of traffic it will generate ?

For now it's only theoretical question, but we would like to estimate
how many machines will it take to handle this kind of load.

Another question how to scale varnish, I'm thinking about setting a 2
loadbalancers whitch will take care of sessions getting to the same
server, and 3x8 core machines for www + varnish or maybe 2x4 core
loadbalancers with varnish and 3x8 core machines for www. I would be
possible to use varnish as a loadbalancer with some http cookie
trickery.

I will be grateful to anyone willing to share his experience.

-- 
Michal Grzedzicki