<p>Bound states in the continuum (BICs) are optical modes that remain decoupled from free-space radiation. Symmetry-protected BICs in metasurfaces offer powerful means to control light-matter interactions. A key property governing these interactions is the partial local density of optical states (PLDOS), which describes the number of electromagnetic modes available for a photon to occupy at a specific position, frequency and polarization. Here, we employ a terahertz near-field microscope with dual local probes to directly excite and detect quasi-BICs in finite metasurfaces that possess inversion symmetry, corresponding to a symmetry-protected BIC in their infinite counterpart. We observe a strong enhancement of the PLDOS associated with these modes. As the metasurface size increases, the quasi-BIC evolves into a true BIC, with the quality factor diverge while the PLDOS saturates. This establishes an upper limit for enhanced light-matter interaction by BICs. Our findings pave the way for on-chip metasurfaces with maximum light-matter interaction strengths.</p>