Help determining Max Capacity of my system

I need some help determining how many active calls my system could support at one time.

i’m running the Free PBX distro 2.6 with asterisk 10.12.0
The server has a GHz processor and 2048 Mb of RAM

That depends on many variables, look first to:-

The limiting factor is often the network bandwidth you have and the codecs you use. For example each g711 call is about 80k each way.