Do I need to increase cooking time when baking many more fish fillets than receipe calls for.
On a Single Rack
A preheated oven will cool down depending on the amount of food put into it, but will lose more heat due to the opening of the door. As long as they are spread out enough, the cooking time will be affected very little.
On Multiple Racks
Assuming you do not have a convection oven: when baking on separate racks of the oven, the heat radiated by the coils is now blocked by the other food. This will increase the time necessary and will also require you to rotate the dishes.