Why the VM does not grow its RAM usuage?

I have a VM running "SBS 2011", hosted by "Windows Server 2012 Standard" Hyper-V role.
Here is the RAM setting for this VM
- Startup: 8GB
- Min. 8GB
- Max: 24GB

Today is the first day to get this VM online. We have over "50" users. However, during the day, I have checked the "Assigned Memory" a few times, it always stayed at 8GB while I expected it to be something close to 24GB.

Please see attached screen shot

The minimum recommended memory for SBS2011 is 8GB. I heard of people giving their SBS 2011 box frequently 12GB+. And I shipped my SBS2011 boxes (not VMs) with 24GB memory all the time to other clients.

So my questions are
1. People are saying SBS 2011 needs much more than 8GB, is this a hype?
2. Is anything wrong with my setting in Hyper-V?
3. I consider "50" users is a quite heavy load on SBS 2011. Why the memory usage stays at 8GB? Should I just leave the settings as is and should not concern about anything?
4. Is there a tool that I can use to trace the memory usage in a VM?

Thanks
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