255 inputs or outputs is the maximum due to the control protocol (ModbusTCP) parsing of the V16Pro. (A single byte at one point in the protocol)
Each input or output refers to 1 of the 4 inputs/outputs on the individual modules that attach to the main control module.
The maximum number of individual modules per main control module would be 63. (4 inputs/outputs * 63 = 252)
The maximum number of main control modules attached to a V16Pro depends on the polling rate of the V16Pro set in WinScriptLive. I would estimate 10 control modules (with up to 252 inputs on each) would be a good approximate maximum number.