Hey,
First of all, only ONE chameleon can be kept per cage. The are solitary creatures which prefer to live in social isolation, outside of breeding attemps.
As for the cage size, it depends on the size and species of the chameleon. There are plenty of places to research proper cage sizes and cage designs on the internet.
The cage should be ALL SCREEN, with no glass what so ever. Glass will just be a perfect breeding spot for bacteria. Use aluminum insect screening(NO fiberlass screening). For the frame, usually standard pressure treated pine works well; we use 2X2's. For the bottom of the cage be sure to use some sort of plastic sheeting, i.e. ployethylene, polystyrene, or pvc sheeting. If you have any other questions be sure to ask.
