 Abstract 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 2665305 VBB 2665305 VBB, centers in hexagonal boron nitride, HBN, with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different defasing sources, we observe more than five-fold improvement in the measured coherence times across all HBN samples. Crucially, we identify that the many-body interaction within the 2665305 VBB 2665305 VBB Ensemble plays a substantial role in the coherent dynamics, which is then used to directly estimate the concentration of 2665305 VBB 2665305 VBB. 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 2665. We are article.tv, links in the description below.