Hi,
1. You can refer to Lync Server User Models to plan your Lync Server
http://technet.microsoft.com/en-us/library/gg398811.aspx
You can also use Lync Planning tool and type 500 in box with the number of users.
The planning tool will then use your answers to recommend a topology based on the tested Microsoft Lync Server user model.
http://www.microsoft.com/en-us/download/details.aspx?id=19711
2. There are two choices for PSTN connectivity:
SIP Trunking
Direct SIP Connections
For details about PSTN connectivity:
http://technet.microsoft.com/en-us/library/gg425749.aspx
Another link for deploying analog devices in Lync environment:
http://technet.microsoft.com/en-us/library/gg398314(v=ocs.14).aspx
Kent Huang
TechNet Community Support
