This app allows you to track in real time the amount of subscribers of a Youtube channel.

Using the app is very simple. Just search by channel's name and select it from the list. In addition to the total number of subscribers the number of posted videos and total of views are also displayed. You can also highlight channels.
The app also has a widget which can be used to track the subscribers of highlighted channel more easily. This widget can be positioned on the smartphone's screen and it will update the number of subscribers of the selected channel every 30 minutes.

It is important to clarify that all the information used by the application is public and made available by Youtube itself.

I hope this app helps you grow your channel. Good luck!