This paper investigates the maximum achievable downlink spectral efficiency of low Earth orbit (LEO) satellite constellations. Spectral efficiency is defined here as the total network sum rate per unit bandwidth per unit area of Earth's surface. To estimate an upper bound on spectral efficiency, the problem is reduced to a single-channel network model, where all satellites and ground terminals operate over a common narrowband frequency channel. Within this model a regular configuration is proposed and evaluated, with satellites and terminals arranged in hexagonal lattices. Numerical results validate that this configuration provides a robust upper bound for spectral efficiency in more complex multi-channel LEO networks, especially when satellite-terminal associations are based on minimum distance. Further improvements are achieved by adjusting association rules to prevent neighboring satellites from simultaneously serving terminals in the same region, highlighting the critical role of interference-aware association strategies.