  This paper presents multiplicity measurements of $K^0_{\textrm{S}}$,
$\Lambda$, and $\bar{\Lambda}$ produced in 120 GeV/$c$ proton-carbon
interactions. The measurements were made using data collected at the NA61/SHINE
experiment during two different periods. Decays of these neutral hadrons impact
the measured $\pi^+$, $\pi^-$, $p$ and $\bar{p}$ multiplicities in the 120
GeV/$c$ proton-carbon reaction, which are crucial inputs for long-baseline
neutrino experiment predictions of neutrino beam flux. The double-differential
multiplicities presented here will be used to more precisely measure
charged-hadron multiplicities in this reaction, and to re-weight neutral hadron
production in neutrino beam Monte Carlo simulations.
