Current requirement in RS485 Circuit
5V*6mA+3.3V*50mA=195mW (total power) only ???


Yes, this is the correct result, in the nominal case.

The short-circuit current is the maximum amount of current that will flow if the driver outputs are shorted (Y/Z outputs). This is not the nominal case. And it will actually go "in place" of the 50mA Vdd2 supply current, so, in case of short circuit, you'll have:

5V*6mA+3.3V*250mA = 855mW


of power dissipated.

Whether you want to account from this in your design depends on the probability to have a short circuit (basically, on who will do the cabling), or whether you absolutely need your design to survice such a case. But since it is not the nominal case, it is up to you to decide (and document whether this is safe or not).