There are certain limits to subscriptions on YouTube. You can only subscribe to a maximum of 2000 channels and you can subscribe to only 75 channels in a day.
However, if you have a lot of subscribers to your own channel, this limit might be different for you. The limit of the channels you can subscribe to grows as your subscribers increase.
YouTube is a dynamic platform, filled with many entertainers. Each person has a specific channel that you subscribe to if you like. However, here are certain limits in place. These include a maximum of 75 subscriptions per day and 2000 subscriptions in total. However, these values change as your own channel grows. If you have more people following you the platform starts increasing the total number of subscriptions you can have.
Usually, as you start becoming a celebrity and rake in more views, YouTube gives you access to its list of elite channels. You can then subscribe to many more people without having the error pop up.