  We investigate whether the considerable diversity in the satellite
populations of nearby Milky Way (MW)-mass galaxies is connected with the
diversity in their host's merger histories. Analyzing 8 nearby galaxies with
extensive observations of their satellite populations and stellar halos, we
characterize each galaxy's merger history using the metric of its most dominant
merger, $M_{\rm \star,Dom}$, defined as the greater of either its total
accreted stellar mass or most massive current satellite. We find an
unexpectedly tight relationship between these galaxies' number of
$M_{V}\,{<}\,{-}9$ satellites within 150 kpc ($N_{\rm Sat}$) and $M_{\rm
\star,Dom}$. This relationship remains even after accounting for differences in
galaxy mass. Using the star formation and orbital histories of satellites
around the MW and M81, we demonstrate that both likely evolved along the
$M_{\rm\star,Dom}{-}N_{\rm Sat}$ relation during their current dominant mergers
with the LMC and M82, respectively. We investigate the presence of this
relation in galaxy formation models, including using the FIRE simulations to
directly compare to the observations. We find no relation between
$M_{\rm\star,Dom}$ and $N_{\rm Sat}$ in FIRE, and a universally large scatter
in $N_{\rm Sat}$ with $M_{\rm \star,Dom}$ across models $-$ in direct contrast
with the tightness of the empirical relation. This acute difference in the
observed and predicted scaling relation between two fundamental galaxy
properties signals that current simulations do not sufficiently reproduce
diverse merger histories and their effects on satellite populations. Explaining
the emergence of this relation is therefore essential for obtaining a complete
understanding of galaxy formation.
