ApplicationIcon Badge Number

You may use BadgeNumber property if you want to set the badge counter for your app similar to the image below:

When the number in this property is 0, the system does not display a badge. When the number is greater than 0, the system displays the badge with the specified number.

See the example below:

using SA.CrossPlatform.Notifications;
...

var client = UM_NotificationCenter.Client;
var content = new UM_Notification();

content.SetTitle("Title");
content.SetBody("Body message");
content.SetSmallIconName("myIcon.png");
content.SetSoundName("mySound.wav");
content.SetBadgeNumber(2);

Platforms 

On Andoird If you want more advanced control for the notification badges, consider using Android API directly:

https://unionassets.com/android-native-pro/applicationicon-badge-number-847

On iOS you may also control application badges number directly:

https://unionassets.com/ios-native-pro/applicationicon-badge-number-609