Hello Sarita Mohankar,
You could use the option of Ganesh.
Or you need to make sure you have a system in place that registrer for every user which notification they read and which they did not read. And based on this system you could count the unread notifications.
So depending on your domain model you need to have an entity that is related to the user with a boolean IsRead and you need an entity with an integer to store the amount of unread notifications.
Hope this helps,
Good luck!