  This chapter describes the assumed specifications and sensitivities for HI
galaxy surveys with SKA1 and SKA2. It addresses the expected galaxy number
densities based on available simulations as well as the clustering bias over
the underlying dark matter. It is shown that a SKA1 HI galaxy survey should be
able to find around $5\times 10^6$ galaxies over 5,000 deg$^2$ (up to $z\sim
0.8$), while SKA2 should find $\sim 10^9$ galaxies over 30,000 deg$^2$ (up to
$z\sim 2.5$). The numbers presented here have been used throughout the
cosmology chapters for forecasting.
