Eric is right,
Thats the one thing I always done here, because it was the requirements of the PLC manufacturer. First they are going to ask how many input/outputs you want. To know the number of inputs, you have to know how many interlocks, switches, buttons, etc. you need. Second, you have to know how many motors you have to run, or other things need to run. Then prepare what you called I/O list. From this, the manufacturer will know how many CPU will be required from the system you submitted.
Like here, mostly we are requesting each area with its own CPU, because you still considered the scan time. Example, in our roughing Area..one set of plc, Intermediate Area, another set, Finishing Area another set again. For Auxilliaries load such pumps, it is another set of plc again.