For applying KVL in a closed loop with more than 1 batteries , do we have to consider the effect of only 1 battery ?.if yes why so??

Even though you may arbitrarily assign directions to the currents, the general rule is to draw currents "coming out" of the positive terminal of a battery. Later, if your choice of direction later turns out to be incorrect, the value of I will simply be negative. However, you must LEAVE it alone since a change of SIGN will result in incorrect values for V and R as you continue to analyze these remaining properties of your circuit