Are we talking about scanning prints or scanning film?
"35 mm film is relatively tiny, requiring greater scanning resolution than photo prints to create an enlarged image for printing. The ratio of (scanning resolution/printing resolution) is the enlargement factor. For example, scanning film at 2700 dpi and printing scaled to 300 dpi gives 2700/300 = 9 times enlargement of the original film size. 9X is about 8x12 inches (near A4 size) from full frame 35 mm. This enlargement requirement is why film needs high resolution.
Most flatbed choices are 600 or 1200 dpi now, and some are 2400 dpi. You won't need more than 300 dpi for scanning photo prints, or 600 dpi for line art documents, assuming printing at original size. 1200 and 2400 dpi would be used for scanning film."