TeslaUnread provides unread count badges for Nova Launcher Prime and WidgetLocker. It will not work with other desktops or lockscreens.

Unread counts are natively supported for the most popular chat and email apps, such as Hangouts, Whatsapp and all the apps shown in the screenshot.
Additionally on Android 4.3+ TeslaUnread can watch the notification bar to extract unread counts from virtually any app.

Enable TeslaUnread in Nova Launcher or WidgetLocker Settings > Unread Counts, here you may also customize the style of the badge, such as the size and colors.
From version TeslaUnread for Nova Launcher 5.1.2:
Improve reliability of Gmail count
Variant
Arch
Version
DPI
(Android 4.1+)
Latest: 4.0.1 on July 16, 2015
(Android 4.4+)
Latest: 5.1.2 on March 29, 2018
All versions