Premium subscription will allow an unlimited number of favorites.
As Series Reminder transitions into having a premium service one of the features that has become limited is the number of continuing favorite series you can have. The free service allows up to 20 Continuing or Upcoming series to be favorited, after which a premium subscription is required to add more. If you are new to the site, there is a 30 day 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 weeks simply cancel your subscription.
There will be no limit on the number of ended series you can add to your favorites. You never know when they might come back. If you are at your maximum number of favorites on a free account, and one of your favorite ended shows returns, you will receive notifications for it and will allow you to go over quota!