The hardware requirements for 3CX Phone System depend on the number of simultaneous calls that you plan to make. Each call makes use of a certain amount of resources, thus the more calls you make at any point in time, the greater the hardware requirements.
The 3CX Hardware Requirements fact-sheet lists the minimum hardware requirements for each edition of 3CX Phone System. Remember that if you are installing 3CX Phone System with other applications, you will need better hardware than what is listed. Alternatively, you can review the Supported Operating Systems for 3CX Phone System here.