you can have up to (10) G700's in a single stack, up to 40 slots.
If you are looking at adding several G700's, at this time, you might be better off looking at the G450, which has 8 slots. Of course you need to upgrade to CM5 to use the G450.
G700's are being End-Of-Lifed, and the replacement is the G450, with 8 slots, capable of having redundant power supplies, up to 240 VoIP resources via plug in MP modules (MP20's and MP80's), replaceable fan cage, replaceable motherboard, in short, a much better media gateway.
You will also find cost wise, the G450 is less expensive by the time you add up the G700, P330 stacking module, and the fact you only get 4 slots, and the voip resources are limited to 64 (32 if you use the G729 codec)
The G450's communicate with each other, and with your existing G700's over IP, which is the same way the G700's communicate to each other (The "stacking module & cable) are only for management, and IP connectivity within the same stack)
Mitch
AVAYA Certified Expert