  We present the analysis of the faint galaxy population in the Advanced Camera
for Surveys (ACS) Early Release Observation fields VV 29 (UGC 10214) and NGC
4676. Here we attempt to thoroughly consider all aspects relevant for faint
galaxy counting and photometry, developing methods which are based on public
software and that are easily reproducible by other astronomers. Using
simulations we determine the best SExtractor parameters for the detection of
faint galaxies in deep HST observations, paying special attention to the issue
of deblending, which significantly affects the normalization and shape of the
number count distribution. We confirm, as claimed by Bernstein, Freedman and
Madore (2002), that Kron-like magnitudes, such as the ones generated by
SExtractor, can miss more than half of the light of faint galaxies, what
dramatically affects the slope of the number counts. We present catalogs for
the VV 29 and NGC 4676 fields with photometry in the g,V and I bands. We also
show that combining the bayesian software BPZ with superb ACS data and new
spectral templates enables us to estimate reliable photometric redshifts for a
significant fraction of galaxies with as few as three filters. After correcting
for selection effects, we measure slopes of 0.32+- 0.01 for 22 < g < 28,
0.34+-0.01 for 22< V <27.5 and 0.33+-0.01 for 22 < I < 27. The counts do not
flatten (except perhaps in the g-band), up to the depth of our observations. We
find that the faint counts m_{AB}> 25.5 can be well approximated in all our
filters by a passive luminosity evolution model based on the COMBO-17
luminosity function (\alpha=-1.5), with a strong merging rate following the
prescription of Glazebrook et al. (1994), \phi^*\propto (1+Qz), with Q=4.
