MMM can record (overdub) up to 64 separate tracks, BUT only one track at a time.
Your question is not completely clear, so please don't take this the wrong way.
Yes, you will record vocals with a microphone.
You should record the voices from your keyboard or synthesizer using direct audio recording though your sound card or audio interface. OR...You could use your keyboard/synth as a MIDI controller using a MIDI or USB interface to play VSTi (software) synths.