Am working on a design project for a commercial office which proposes to install 400 computers. Please advise how do I determine the maximum demand in amperes from the computer load in order to select the correct rated cable size

You would need to know what computers they are going to use and then add some extra to the circuit to prevent local brown outs. Also your going to need to know what else they may be using with them and calculate that into the load. Monitors and printers etc.