Because of the sheer mass needed to make a hypergiant (around 100 solar masses), along with their lifespan of just a few million years, they would be among the rarest stars out there.
That's funny more or less because hypergiants aren't really based on luminosities but rather on their diameters.
For example a 100 Solar mass star would likely be refered as luminosity class Ia-0 (extremely luminous supergiants).
They're also commonly known as LBV's (luminous blue variables) like the Eta Carinae or the Pistol Star.
That's due to their tremendous brightness but their relatively ''small'' diameters.
A hypergiant is based on diameters and therefore even a class Ia / Iab / Ib could be refered as that and would have a mass of atleast 20 - 25 Solar masses.
Interesting note: the yellow-white hypergiant Rho Cassiopeae actually zig-zag crosses the ''yellow hypergiant'' region in the HR diagram as fast as 50.000 - 100.000 years (probably less) ...