It depends on how you show the User the “Items” that can be favorited. Let’s say these are Songs…
You don’t have to store all the Favorites in one single list.
For Example:
You can use another object that filters by the Current User + The Artist then lists all the songs liked and you will likely never reach thousands of items in a list. Break it down further based on your scenario.
*Artist-Favorites
- User (single)
- Artist (single)
- favorited_songs (list)
If this is an Instagram App where you want to show the amount of likes of an Item, then you take a different approach.
Storing the data in a manner that the User can arrive to it in the same way they arrive to the assets on screen usually helps.
Don’t Forget: You may need to break a few rules 