transfer resolution of 4x6 photo prints for preservation?

I couldn’t find much info on this and I’ve got a bunch of old family photos i want to digitize so I thought Id ask here. What is the practical max resolution of an old school 35 film photo print? I know film varies from case to case but at want point would I just be getting wasting time? my scanner maxes out at 1200 dpi which if I did my math right is about 20 megapixels.