For a project this large, a phone app is probably not the best workflow. It may seem simple, but photographing 20,000 frames individually with a phone is likely to be slow, repetitive, and difficult to keep consistent.

The better options suggested were:

• outsource the job to a good photo lab, which could produce both usable scans and contact-sheet-style references in one pass, or
• use a dedicated film scanner, ideally one with an automated feed or holders that let you prepare the next strip while scanning.

A dedicated scanner is especially sensible for 35mm negatives if they’re already cut into strips. It will usually give you more consistent framing and easier file handling than a phone app.

Before choosing a method, consider how the negatives are stored and cataloged. If they’re in sleeves, numbered, or already organized, that may affect whether you can scan them quickly without extra handling and how easy it will be to retrieve frames later for high-quality rescans.

In short: for 20,000 negatives, prioritize a repeatable workflow. A lab is likely fastest overall; a dedicated film scanner is the best DIY option. A phone app is possible, but probably the least efficient at this scale.