Brining multiple chickens
If you stay with the same ratio of salt (and any other ingredients) to water that you normally use, you should achieve the same results. As you normally use a 5% solution it shouldn't matter how many chickens or how much water if you stay with the 5% solution.

The only thing that the weight of the chickens will factor into is the amount of brining time needed. A larger bird will take more time than a small one.

Re time, a good rule of thumb for whole chickens is 2 hours per pound, using a 4% solution. You may want to slightly reduce the time per pound using a 5% solution. So, your 5 hours sounds about right. I wouldn't go over that as I would be concerned that it may make the meat mushy. (Haven't ever done that but I've heard that it can happen if over brined.)