We investigate whether the considerable diversity in the satellite populations of nearby Milky Way (MW)-mass galaxies is connected with the diversity in their hosts merger histories. Analyzing 8 nearby galaxies with extensive observations of their satellite populations and stellar halos, we characterize each galaxys 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 $M_{rmstar,Dom}{-}N_{rm Sat}$ 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 $M_{rmstar,Dom}$ 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.
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