We usually scan black and white photographic prints on flatbed
scanners at 600 dpi, 8-bit grayscale. 24-bit color could be as low as
400 dpi.

NARA suggest achieving 3000 pixels on the long side of photo images.
Later thinking is to increase that to 5000 pixels on the long side. If
scanning a variety of materials, from slides to 8 x 10s, this is a
better way to think about what is needed to achieve the detail
wanted. See this brief, helpful article that summarizes more recent
thinking.

Finally, testing with a few representative images is always a good
idea before entering full production.