The free service allows up to 20 series to be favorited, after which a premium subscription is required to add more. If you are new to the site, there is a free trial that you can cancel any time. Take a minute to setup and add as many shows as you remember enjoying. If you don't like it after a few days simply cancel your subscription.
The maximum number of favorites is simply the number of series you have favorited, regardless of the series status (Continuing or Ended). Add the shows your currently watching and waiting for the most first. But, don't forget to add some of your favorite shows that are ended too. You never know when they might come back. More and more, studios are rebooting beloved past series with a new season. An unexpected notification makes for a pleasant surprise once in a while.
If you have been a Series Reminder user for a while now, there is a chance that you have more than 20 favorites. If you do, you will no longer be able to add any new favorites unless you become a premium member. However, you will still be able to manage the individual series notification settings for all of your existing favorites. If you remove a favorite, you wont be able to add it back again unless you get below 20 favorites or upgrade to premium. Please support the site.
Originally I was going to try to make the limit based on the number of Continuing series you have in your favorites list. This would have allowed unlimited Ended series. Ultimately, I decided against it as it makes the boundary more simple to think about and code.