Virtualization

Depends on the hardware. What are you running it on?

I dont have any server yet..

an SMS server han can host tens of thousands clients..

So lets say with an new server with two CPU with lots of memory..
Is it thousands of clients or more or is it just the hardware how set the limit.

/Tjabocom

One SMS server providing all roles couldn't serve tens of thousands of clients on its own.

They say a Sgrid server can host about 800 users at a time with moderate specs. You'd want at least two servers for failover. Use a load balancer.
With two brand new servers with excellent specs, I'd say you could safely host 2000 users.

Kkamisk could give you more real-world experience in this matter.

The load is really going to depend on how much streaming will occur and if you are going to be using feature block one. Having 800 users per CPU is a good ballpark and having redundancy is not required but a nice to have especially with a large desktop deployment. Usually the server sits back and collects useage information but the real load will come from streaming either due to feature block requests, upgrades and new application deployments.