The only info I can offer is each BRAKE MAGNET will draw around 3.0 DC AMPS each with 12VDC applied to it...
You should also check with your state laws on what you can tow with one truck where your trailer is licensed... Add all of the wheels up that has electric brakes installed and see if your planned brake controller is capabile of providing that much current.
Alot of commercial brakes on similar utility trailers have SURGE BRAKES instead of ELECTRIC BRAKES. This may be a solution for you to check into as well.
Roy Ken