I manage many channels, but having them under one CMS makes it much much easer and fast. I have my own CMS, but there is a limit of adding 20 channels per year as outlined here:
support.google.com/youtube/answer/106934?hl=en
Keep in mind that having so many channels under one Adsense account makes them prone to demonetization all at once, if one of them goes south for whatever reasons. Managing 20+ separate Adsense accounts and 20+ YouTube channels will be next to impossible task for single person.
Private message me if you need help or more information about CMS/Multiple Channel Networks and managing them.