Short answer: you have to use a microscope, a drum scanner or some specific scanners for film.
Flatbed scanners barely reach 1500 dpi (real, effective, measured on the details you get, not on the number of pixels you get), see http://www.filmscanner.info/EpsonPerfectionV600Photo.html and the other ones.
This is a measurement of actual details in some 120 film (provided you focused perfectly!): https://www.onlandscape.co.uk/2014/12/36-megapixels-vs-6x7-velvia/
Summary of the article: if you consider that Adox with over effective 200 Mpx is a special film you don't use everyday (but there is no way to scan that many pixels), you can extract reasonable details at 4000-5300 dpi from generic 120 film, equivalent to 50-80 Mpx.
If you didn't focus perfectly, divide by two in both axes and here are the 20 Mpx scans, provided they are performed with a GOOD scanner, not a normal flatbed, even if marketed for photos at more than 2400 dpi. No home-hoffice (<500 dollars/Euro) flatbed scanner can do that. Do not trust specifications, check the technical reviews.
Above that price point, two scanners available to a home user reach an effective resolution of over 4000 dpi and can get most out of 120 film: filmscanner.info/ReflectaRPS10M.html and filmscanner.info/NikonSuperCoolscan9000ED.html
Keep in mind: I used the Nikon one and I could see the grains in the center, but not on the sides: using glass plates to keep the film flat is a must.
As a matter of fact, you may even use an high-end dSLR like D8x0, provided you have a reliable way to keep the film flat (meaning the specific glass plates), you autofocus well, keep the film perpendicular and keep lighting uniform. A D800 produces pictures of 120 film with about 24 Mpx. It would be optimal. Check here and adapt the distance between camera and film: http://petapixel.com/2012/05/18/how-to-scan-film-negatives-with-a-dslr/