You can use a non persistent object for that with a microflow as source. In the microflow do a retrieve and a count list, create the non-persistent object and return that object.
[EDIT] You can not display this value in navigation. You should place it somewhere on your home screen of this user (the first screen the user sees when he opens the application). On this form you can create this info.
Rapido, take a look at this post for how to style a counter in the toolbar: