Lightroom catalogs can handle very large libraries, but performance usually declines as they grow. Adobe’s own guidance for Lightroom 3 said there is no fixed photo limit, though practical limits may be reached somewhere between about 100,000 and 1,000,000 photos depending on the computer.

Real-world reports in the discussion say a catalog with about 211,000 images was still usable, though slower than small catalogs and notably slower to start. Startup could take a few minutes while Lightroom scanned or validated the library, but the interface remained responsive enough to work during that process.

So a single 500,000-image catalog is not necessarily impossible, but you should expect slower startup and potentially slower searches or other database-heavy operations. Smaller catalogs generally perform better.

If simplicity is your priority, one catalog may still be workable on a strong system. If speed is critical, keeping multiple catalogs can remain the better tradeoff. Also, trimming unneeded images can reduce both storage and catalog-management overhead.