  In the past fifteen years a new generation of X-ray satellites led to the
discovery of a subclass of high-mass X-ray binaries (HMXBs) with supergiant
companions and a peculiar transient behaviour: supergiant fast X-ray transients
(SFXTs). We calculate the expected number of Galactic SFXTs for the first time,
using two different statistical approaches and two sets of data based on Swift
and INTEGRAL surveys, with the aim to determine how common the SFXT phenomenon
really is. We find that the expected number of SFXTs in the Galaxy is about
37(+53, -22) which shows that SFXTs constitute a large portion of X-ray
binaries with supergiant companions in the Galaxy. We compare our estimate with
the expected number of Galactic HMXBs predicted from observations and
evolutionary models and discuss the implications for the nature of SFXTs.
