It depends upon some calulation as given below :
1.How much memory has been used by first user ? =A
It will take slightly more memory as .qvw needs to be loading first time in memory.
2.How much memory has been used by second user after .qvw file is in memory (it will be less than first user) =B
3. As you mention that you have 8 GB of ram then its handling limit should be around 7.2 GB because after this server become unresponsive and it will get hung.
4. User = [(7.2 - A) / B ] +1
You can measure memory from task manager.
Note--> it will not come 100% accurate because if you have shared/collabration objects the the value of memory usage will be differ User wise.
It's hard to say how many users you can run on an 8GB machine for a generic 500MB document. But since you have the document, you can easily run a test by first loading the document from the QVS and look at the RAM usage. If QVS now takes up 1GB RAM and you add another user, you might see the RAM usage increase with about 5-15% depending on how the document is structured. At this point it scales up pretty linearly with every new user adding a similar overhead.