Depends on the diameter of your carboys and the usable flat surface area of your fridge.
A single door, used, flat bottomed commercial fridge like the kind that holds flowers at the grocery store, or professional film at a photo store, or softdrinks and beer at your local Deli would probably accommodate six bottles, three on the bottom two on a sturdy shelf you build above it. Or perhaps you could fit two 27 gallon conicals side by side in a double-door model. These sorts of refridgerators are often available for reasonable amounts of money if you look around at store and restaurant closings and such.
Your typical home refridgerators come in all sorts of sizes and configurations. Many do not have flat bottoms, so you might need to build a shelf at floor level (plywood) to make a flat surface. My larger refridgerator is tall and narrow and could only handle two bottles. My smaller refridgerator is squat and wide and could hold three.