You'll probably never exceed the weight limit of a rack with surfboards, but you can easily exceed the aerodynamic load limit, which in the case of many factory racks secured with sheet metal screws is close to zero.
If you stack many boards on a rack it's a good idea to tie down the noses and add a through strap that runs over the boards and through the doors. Slam the doors on the straps and you're fine unless it rains. The rain will follow the strap through the weatherstripping, run along the strap and then drip unerringly in your crotch. This is known.