Quantity limits: No limit on devices. Two spare fuel cell cartridges may be carried.
Fuel cell cartridges may be changed out in the device but empty fuel cells and fuel cell cartridges may not be refilled with fuel while on the aircraft.
Each fuel cell must also be marked "Approved for carriage in aircraft cabin only".
Each fuel cell cartridge (not the electronic device) must be marked with the maximum quantity and type of fuel in the cartridge.
See 49 CFR 175.10(a)(20) for fuel cell size limits and other requirements.
For international flights check with your airline as international regulations may vary from domestic regulations.