If your folder count matches 21,866, you know your download or transfer was successful without corruption.
Instead of using a "portable" bundle, download images manually and use a trusted, open-source viewer like ImageGlass kuni scan complete collection 21866 pics portable
Large-scale scans like these serve as a digital backup for physical media that may degrade over time. How to Manage a Collection of This Scale If your folder count matches 21,866, you know