I will admit I used to carry fert in my cab. rain not a problem. Its a whole lot dryer there. 10 bags if you really try. And if you do it right you can still see out the side mirror. Be careful, though, drive sedate. There is a chance a couple bags could fall out the open window. And there is a chance that a tall stack of bags could fall towards the unfortunate driver--pinning him against the driver side door.
In fact, when I worked for Trygreen, a few times when eating lunch at a restaurant...I rassled my spreader upside down into the cab. Kept it dry and prevented theft.