  We present results from the remaining sources in our search for near-infrared
(NIR) candidate counterparts to ultraluminous X-ray sources (ULXs) within
$\simeq$ 10 Mpc. We observed 23 ULXs in 15 galaxies and detected NIR candidate
counterparts to six of them. Two of these have an absolute magnitude consistent
with a single red supergiant (RSG). Three counterparts are too bright for a RSG
and spatially extended, and thus we classify them as stellar clusters. The
other candidate is too faint for a RSG. Additionally, we present the results of
our NIR spectroscopic follow-up of five sources: four originally classified as
RSG and one as a stellar cluster on the basis of previous photometry. The
stellar cluster candidate is actually a nebula. Of the four RSGs candidates,
one source has a broad H$\alpha$ emission line redshifted by $\sim z = 1$,
making it a background AGN. Two other sources show stellar spectra consistent
with them being RSGs. The final RSG candidate is too faint to classify, but
does not show strong (nebular) emission lines in its spectrum. After our search
for NIR counterparts to 113 ULXs, where we detected a candidate counterpart for
38 ULXs, we have spectroscopically confirmed the nature of 12: five sources are
nebulae, one source is not classified, one source is an AGN and five are RSGs.
These possible five ULX-RSG binary systems would constitute $\simeq$ $(4 \pm
2)\%$ of the observed ULXs, a fraction almost four times larger than what was
predicted by binary evolution simulations.
