Accu (Battery)

The energy density (watt-hours per kg) is dependent on the technology used.
Right now the best (or almost the best) is lithium-polymer, or lipo, so that's why we're all using it. It gets better with time as the technology progresses.
If you have more motors you have to consider the battery ratings: a 5000 mAh 20C battery has a max current of 5*20 = 100 A.
If you have eight motors and each can draw up to 17 A (136 A total), Then you either need to use a stronger battery (or more of them in parallel) or limit the engines in software using a current monitor.