One voice. That's all any good singer needs. It's the amateurs who try to sing in multiple ways.
When Johnny cash sings he doesn't sound like Prince, then Paul McCartney, then Frank Sinatra does he? Of course not. EVERY time he sings he sounds like Johnny Cash. 99.99 % of all successful singers sound like THEMSELVES. And 99% of that 99.99% sing very close to the same way, song after song after song after song.
Amateur wannabes are different though. They try, in vain, to sing like their favorite singer in one song, then like another different singer in another song, then they hear someone singing high falsetto and they have to sing like that too, then they hear a great baritone so they have to sing like that too, then they hear someone singing softly with great success so they then must sing softly, but later on they hear someone singing loudly with great success so they think they then have to master loud singing etc etc etc.
Pretty much all the famous, really good and successful singers ... sound like THEMSELVES, and sing in their OWN unique voice.
This is partially true but is also completely wrong. A lot of the people you mentioned are amateur singers who happened to get successful.
- The fact that these people are limited by their own sound/choose to only use their own sound means they have a distinctive 'brand' as a popstar.
- As a session vocalist... well... versatility is a must. Good session vocalists are really far more professional than the people you mentioned - Johnny Cash in particular.
Another point - Lana Del Rey is also quite successful and has at least 3 voices on record: high girly; low sultry and 'rap' voice.