You only use one at a time. Should fit most battery compartments. Check size: 125mm(L) 22mm(W) 13mm(H)
Depends on motor size and how much firing you do but 2 should last a day.
NOTE: The connector is NOT standard. If you have not changed your weapon connector to DEANS then look for same batteries with TAMIYA connector instead.
You'll also need a good LiPo charger. Search the forum for more info