Panels (c) and (d) in Fig. correspond to the case for N = 10 molecules. Since the linear response signal is a direct measurement of the coherent oscillations between the upper and lower polariton states, compared to panel (a), we can observe an increase in the coherence lifetime In Fig. , we present the convergence of absorption lineshape with an increasing number of molecules from N = 1 to N = 10 5 , while fixing the collective Rabi splitting, ℏΩ R = 2 √ N ℏg c = 0.2 eV. We use a cavity loss rate of Γ = 20 meV and the same molecular parameters as used in Fig. . Fig. presents the lower polariton lineshape for a different number of molecules. The LP spectra with N = 10 2 molecules (green curve) are visually close to the converged lineshape of N = 10 5 molecules (thin dark blue curve). From these curves, the lineshape features of linear spectroscopy due to collective effects appear to reach a convergence at N ≈ O( 2 ), which is very much closer to the lineshape features of N = 10 5 . The theoretical approaches of this work allow us to directly perform the quantum dynamics and linear spectra simulations with N = 10 5 molecules and explore various trends of convergences.