You always need to leave some space empty, at least 10-15% in order to keep performance unaffected.
After OS and software installation of which most people have quite a bit after office, benchmarking tools, anti-virus etc. 120GB (100GB usable) is not going to be enough.
1000 PCs in a network running off a single 128GB drive?
Not possible, I think you may have worded that incorrectly, also I assume you mean 1000 VMs, not physical terminals since that's an impossibility too.
You'd be running enterprise level storage at that stage, and would be very unlikely to be using purely SSD drives since you'd need to be a multi-million dollar company to afford that expense obviously.
This is in regards to a personal rig anyway, the needs are different.