Default
Light
Dark
With Border
1. Numbers
Counters should only contain numbers.
2. Usage
Counters should have a clear connection to the non-visible items they represent.
3. Count Cap
Counters should have the option to cap the number count. And "+" should be use to indicate additional items.
Additional Reading
Indicators, Validations, and Notifications: Pick the Correct Communication Option https://www.nngroup.com/articles/indicators-validations-notifications/