How many uploads can my server handle at once?

[eluser]esset[/eluser]
I'm running a 4CPU, 4GB ram VPS server with a codeigniter installation on it. On my site people can upload files. I'm wondering if anyone have any idea how many uploads my server can handle at once before it starts to freak out?

Can it handle 100 user uploads (1mb/file) at once?
1000 users at once?
10,000 users at once?

Anyone know how to do the math for this?

Thanks guys