Okay, using the base form of the spell, and as stated above that it requires 2' worth of BMR to get through 1' thickness of Air Wall.
It doesn't matter how many Air Walls are placed side-by-side, or how many times the scaling option, Increase Size is used to make the Air Wall bigger. It still requires 2' worth of BMR to move through 1' of Air Wall.
As for the Penalties for attacks through the Air Wall, using multiple Air Walls, I would say to increase the penalty by half of the value of each additional Air Wall (i.e. 2 side-by-side would == -75, NOT -100; 3 Air Walls would be -100 (note: using the Increase Churning scaling option to increase to 100% would take just as long to cast and cost 2 PP less in this manner))