  Ultra-diffuse galaxies (UDGs) are very low-surface brightness galaxies with
large effective radii. Spectroscopic measurements of a few UDGs have revealed a
low dark matter content, based on the internal motion of stars or globular
clusters (GCs). This is in contrast to the large number of GCs found for these
systems, from which it would be expected to correspond to a large dark matter
halo mass. Here we present Hubble Space Telescope Advanced Camera Survey
observations for the UDG MATLAS-2019 in the NGC5846 group of galaxies. Using
images in the $F606W$ and $F814W$ filters, we trace the GC population two
magnitudes below the peak of the GC luminosity function. Employing Bayesian
considerations, we find a total of 37$\pm$5 GCs associated with the dwarf,
which yields a large GC specific frequency of $S_N=84\pm 12$. Due to the
superior image quality of the HST, we are able to resolve the GCs and measure
their sizes, which are consistent with the sizes of GCs from Local Group
galaxies. Using the linear relation between the total mass of a galaxy and the
total mass of GCs we derive a halo mass of $1.3\pm0.2\times10^{11}$ M$_\odot$,
corresponding to a mass-to-light ratio of over 1000. This suggests that either
this UDG has an overly massive dark matter halo for its stellar mass, compared
to other dwarfs -- though not as massive as the Milky Way -- or that the linear
relation between the number of GCs and the dark matter halo mass breaks down
for UDGs like MATLAS-2019. The high abundance of GCs, together with the small
uncertainties, make MATLAS-2019 one of the most extreme UDGs, which likely sets
an upper limit of the number of GCs for such objects.
