  The maximal speed $v^\ast$ for magnetic flux quanta is determined by the
energy relaxation of unpaired electrons and is thus essential for
superconducting microstrip single-photon detectors (SMSPDs). However, the
deduction of $v^\ast$ from the current-voltage ($I$-$V$) curves at zero
magnetic field is hindered by the unknown number of vortices, $n_\mathrm{v}$,
as a small number of fast-moving vortices can induce the same voltage as a
large number of slow-moving ones. Here, we introduce an approach for the
quantitative determination of $n_\mathrm{v}$ and $v^\ast$. The idea is based on
the Aslamazov and Larkin prediction of kinks in the $I$-$V$ curves of wide and
short superconducting constrictions when the number of fluxons crossing the
constriction is increased by one. We realize such conditions in wide MoSi thin
strips with slits milled by a focused ion beam and reveal quantum effects in a
macroscopic system. By observing kinks in the $I$-$V$ curves with increase of
the transport current, we evidence a crossover from a single- to multi-fluxon
dynamics and deduce $v^\ast\simeq12\,$km/s. Our experimental observations are
augmented with numerical modeling results which reveal a transition from a
vortex chain over a vortex jet to a vortex river with increase of
$n_\mathrm{v}$ and the vortex velocity. Our findings are essential for the
development of 1D and 2D few-fluxon devices and provide a demanded approach for
the deduction of $v^\ast$ at the SMSPD operation conditions.
