  The number density and correlation function of galaxies are two key
quantities to characterize the distribution of the observed galaxy population.
High-$z$ spectroscopic surveys, which usually involve complex target selection
and are incomplete in redshift sampling, present both opportunities and
challenges to measure these quantities reliably in the high-$z$ Universe. Using
realistic mock catalogs we show that target selection and redshift
incompleteness can lead to significantly biased results, especially due to the
flux limit selection criteria. We develop a new method to correct the flux
limit effect, using information provided by the parent photometric data from
which the spectroscopic sample is constructed. Our tests using realistic mock
samples show that the method is able to reproduce the true stellar mass
function and correlation function reliably. Mock catalogs are constructed for
the existing zCOSMOS and VIPERS surveys, as well as for the forthcoming PFS
galaxy evolution survey. The same set of mock samples are used to quantify the
total variance expected for different sample sizes. We find that the total
variance decreases very slowly when the survey area reaches about 4 deg$^2$ for
the abundance and about 8 deg$^2$ for the clustering, indicating that the
cosmic variance is no longer the dominant source of error for PFS-like surveys.
We also quantify improvements expected in the PFS-like galaxy survey relative
to zCOSMOS and VIPERS surveys.
