A large portion of the scenes that the Forsaken are actually in cover fairly important exposition and/or provide essential insight into the Dark One's character and honestly if I were in charge of the show I'd be tempted to add more of them. However, I do agree that 13 characters is simply too many to handle in any substantial way. Still, having high-powered baddies is useful for a long-running show, so I think it makes more sense to organize the Forsaken into two tiers: actual characters and disposable obstacles. The books already do this to a degree and really its just a matter of shifting a few over to the disposable side and giving the handful of substantial things they actually do over to others.
Of course, this is another late developing issue. In the first three books only one of the Forsaken - Lanfear - functions as an actual character. Aginor, Bathamel, and Be'lial are completely disposable baddies and Ishamel is functionally a mobile plot/exposition device who barely manages to operate as a person most of the time.