Assuming you are stacking the magnets on top of each other (so they are in attraction), the output of the stack will be higher than the magnets individually. The amount of increase depends on the size of the individual magnets. However, a stack of two magnets will not double the output, it'll work out to be less than 2x. Similarly, a stack of three magnets will have less than 3x the input of an individual magnet.
When stacked, neodymium magnets behave pretty much as a single solid magnet.
Best regards,
Mike