Question: Just how many microphones do you want to record an individual human voice? Only one.
For classical music you may want to use more microphones to recapture the ambience of the recording space, but also for every other sort of recording then you definitely only need one microphone for an individual human voice.
It's worth saying here that for just about any single instrument you may use two mics to produce a stereo recording. But an individual human voice doesn't have useful stereo content, so one mic is okay, and it'll capture a well balanced image where two mics may not.
So you will need to possess just one single mic, yes?
Well yes. When you have a microphone of basic professional quality, you are able to record an individual human voice to an expert standard. Obviously you will need good sound insulation and acoustic treatment in your studio, but a microphone just like the Shure SM57 or better has the capacity to deliver professional results.
I'd go further and claim that there isn't every other microphone, regardless of how expensive, that may make work more professional. Just different.
And that's the point: different.
Different microphones have different sound characteristics. So you may wish to have a microphone that captures a really natural sound (like the DPA 4011), or perhaps a microphone with a vintage neutral sound (Neumann U87), or perhaps a microphone that flatters the singer (AKG C12 - You want!), or perhaps a microphone that's the classic rock sound (back to the Shure SM57).
Also, different microphones suit different singers. A microphone that sounds good using one singer mightn't sound so good on another, the place where a different mic could be better. This may vary from daily so it's always worth establishing two mics and seeing that is best before opting for a take.
Therefore the quick conclusion to that is that all that's necessary for professional results is one mic of at the very least basic professional quality.
But when you intend to optimise the sound of one's vocal recording, then having a collection of mics available is better.
Yet another thing...
I said from the beginning of this informative article that I would look at a single human voice.
Well this covers speech along with singing, and there's an essential difference.
The priority in singing is that the recording sounds good. The priority in speech is that the recording (or broadcast) is easily intelligible. You may look at a mic that's noted for its suitability for speech including the Electro-Voice RE20 or Coles (STC) 4038.RE20 or Coles (STC) 4038.