A standard HD (high definition) computer monitor has 1,920 pixels across and 1,084 pixels in height. (PPI = Pixels Per Inch). Standard digital TVs are now "4K" and higher are 3,840 pixels per inch across and 2,160 pixels per inch in height. An Apple iMac 5K screen has 5,120 pixels across and 2,880 pixels in height. Why is this information important? If you are going to display your photos on a television screen, you need to know at what resolution you will scan your photos at.
For most people just preserving photos to use in a book, or to share with others, the standard "HD" resolutions of 1,920 PPI across X 1,084 PPI in height should be sufficient to give you good printing quality.
The following recommendations are "general" recommendations only. They will at least give you a starting point to begin your scanning.