Is it necessary to make the exact number of turns in each arm of the armature of a motor
Yes it is necessary because you don't want the weight of each arm to be unbalanced, depending on the required number of turns on each arm, adding 2-5 more turns wont really hut any body, for example, if the number of turns are 400 on each arm, then it is safe to have a difference of 2-5 turns.