Your answer comes from: MEMU - Wikipedia > MEMU trains are designed for semi-urban and rural areas, unlike EMU (electric multiple unit) trains that are designed for urban and semi-urban areas. read more
In EMU's 1 power car is required for 3 coaches so for an EMU 12 coaches in length will have 4 power cars. read more
MEMU is Mainline Electric Multiple Unit which runs on mainline generally for distances more than the suburban sections. But there are some exceptions to this definition as can be seen in the NDLS-MTJ section where 2 EMU's run daily between SSB-MTJ which is beyond the suburban section. Same is the case with NDLS-ALJN EMU. read more
Now coming to the difference between an MEMU and a DEMU. In an MEMU, the primary source is electrical power, collected from the overhead catenary. While in a DEMU, the primary source is diesel. A diesel engine powers an alternator and subsequently feeds the electric motors. read more