An air popper off the shelf will eventually fail since coffee roasts at a much higher temp than for roasting popcorn. If you want to increase your quantity and not break the bank the Zippy pop is worth looking at and is similar to the Whirley pop. I had a Whirley but it was not dependable and I don't use aluminum cookware. I roast on an electric hot plate and so 4 cups per roast. It all depends upon how much coffee you use and whether you have a good place to deal with the smoke. Hot air guns work too and that is the same concept as the air popper. If you can get a chamber similar to teh one is your air popper you can create you own heat gun popper. The SS Zippy has gone up in price but has wide paddles to move the beans and a larger opening to monitor the roast. I use an oven thermometer so I know when it is hot enough to start. Listen for the cracks and watch your beans. You could incorporate a thermometer for the roasting but with experience you really don't need one. Sweet Maria's has some good information about home roasting. Good luck.

I roast with an original west Bend Poppery in very cold temps with no problem. And you must do the thermostat bypass to get good roasting temps. Hot air poppers are not created equal. The Poppery is the best, but getting harder to find at a decent price.