  This Letter presents a differential cross-section measurement of Lund subjet
multiplicities, suitable for testing current and future parton shower Monte
Carlo algorithms. This measurement is made in dijet events in 140 fb$^{-1}$ of
$\sqrt{s}=13$ TeV proton-proton collision data collected with the ATLAS
detector at CERN's Large Hadron Collider. The data are unfolded to account for
acceptance and detector-related effects, and are then compared with several
Monte Carlo models and to recent resummed analytical calculations. The
experimental precision achieved in the measurement allows tests of higher-order
effects in QCD predictions. Most predictions fail to accurately describe the
measured data, particularly at large values of jet transverse momentum
accessible at the Large Hadron Collider, indicating the measurement's utility
as an input to future parton shower developments and other studies probing
fundamental properties of QCD and the production of hadronic final states up to
the TeV-scale.
