  We study the effects of strong lensing on the observed number counts of mm
sources using a ray tracing simulation and two number count models of unlensed
sources. We employ a quantitative treatment of maximum attainable magnification
factor depending on the physical size of the sources, also accounting for
effects of lens halo ellipticity. We calculate predicted number counts and
redshift distributions of mm galaxies including the effects of strong lensing
and compare with the recent source count measurements of the South Pole
Telescope (SPT). The predictions have large uncertainties, especially the
details of the mass distribution in lens galaxies and the finite extent of
sources, but the SPT observations are in good agreement with predictions. The
sources detected by SPT are predicted to largely consist of strongly lensed
galaxies at z>2. The typical magnifications of these sources strongly depends
on both the assumed unlensed source counts and the flux of the observed
sources.
