  Optically active spin defects in van der Waals materials are promising
platforms for modern quantum technologies. Here we investigate the coherent
dynamics of strongly interacting ensembles of negatively charged boron-vacancy
($\mathrm{V}_{\mathrm{B}}^-$) centers in hexagonal boron nitride (hBN) with
varying defect density. By employing advanced dynamical decoupling sequences to
selectively isolate different dephasing sources, we observe more than 5-fold
improvement in the measured coherence times across all hBN samples. Crucially,
we identify that the many-body interaction within the
$\mathrm{V}_{\mathrm{B}}^-$ ensemble plays a substantial role in the coherent
dynamics, which is then used to directly estimate the concentration of
$\mathrm{V}_{\mathrm{B}}^-$. We find that at high ion implantation dosage, only
a small portion of the created boron vacancy defects are in the desired
negatively charged state. Finally, we investigate the spin response of
$\mathrm{V}_{\mathrm{B}}^-$ to the local charged defects induced electric field
signals, and estimate its ground state transverse electric field
susceptibility. Our results provide new insights on the spin and charge
properties of $\mathrm{V}_{\mathrm{B}}^-$, which are important for future use
of defects in hBN as quantum sensors and simulators.
