  Dense aperture arrays provide key benefits in modern astrophysical research.
They are flexible, employing cheap receivers, while relying on the ever more
sophisticated compute back-end to deal with the complexities of signal
processing required for optimal use. Their advantage is that they offer very
large fields of view and are readily scalable to any size, all other things
being equal. Since they represent "software telescopes", the science cases
these arrays can be applied to are quite broad. Here, we describe the
calibration and performance of the AARTFAAC-12 instrument, which is composed of
the twelve centrally located stations of the LOFAR array. We go into the
details of the data acquisition and pre-processing, we describe the newly
developed calibration pipeline as well as the noise properties of the resulting
images and present radio source counts at 41.7 MHz and 61 MHz. We find that
AARTFAAC-12 is confusion limited at 0.9 Jy/PSF at 61 MHz with a PSF size of
17'x11' and that the normalized source counts agree with the scaled VLSSr and
6C survey counts. The median spectral index of the sources between the two
frequencies we observed at is -0.78. Further, we have used the derived source
counts to estimate any excess cosmic radio background, and we do not find
evidence for it at our observing frequencies compared to published literature
values.
