we would like to know the modbus communication distance maximum.
Modbus itself specifies no distances, as it doesn't even specify any particular physical layer. Modbus can be carried over any media, such as RS-485, RS-232 etc., and the distance limitations would then be the ones particular to that physical layer.
He's the kind of guy, that, well, if you were ever in a jam he'd be
there ... with two slices of bread and some chunky peanut butter.