How can i externally power 5 micro servos and one mg996r?
The MG996R draws 2.5 A at 6 V. SG90's draw about 0.75 A peak each, for a total of up to 3.75 A peak for 5 servos. Therefore you need a battery that can deliver ~6 A.
A 4 cell (4.8 V) AA Eneloop pack would probably do it, but 2 packs in parallel would be better. If you use individual cells in battery holders the springs will drop more voltage than the welded joints in an R/C receiver pack, and then you will definitely need two 'packs'. Another option might be one 4.8 V pack for the SG90's, and another 6 V pack for the MG996R.