The narrations say that there have been 124,000 prophets sent to mankind, which I take to mean 'an uncountable number'. The Qur'an says 'some of them you know, and some of them you don't know', so there have certainly been more than a selected few in the history of humanity.
With every station (or 'privilege') comes responsibility. How would you have felt having to face the army of Yazid, with the full responsibility of keeping the true Islam alive for humanity and for ever after?
Many people envy other people's positions or stations, but don't realise the intense difficulties and burdens that go with the station.
The Imams have said that it is possible for other human beings to come awliya'. The station of a waliyullah is very close to that of an Imam (see Usul al-Kafi), but again, it requires solid self-discipline and sacrifice.