I tried to bake at the same time 4kg of bread, split into 8 loafs of 500g, 4 breads per rack, it was packed in there! I set the oven at 500F, the top rack definitely got more steam that the bottom one. As a result, and I am not sure why, the breads on the top rack turned out better.

Next time I should try to insert the racks in a specific order, may be increase the temperature too.