In My Products list, add your product and click on Enable hijack tracking icon on the right:

Or, you can add your product from Hijack Listing tool:

And that is it. From this moment, you'll receive an email alert in real-time once hijacker activity is detected on any of your Amazon listings.

To see the list of all changes in number of sellers:

1. click on the product name to expand it.
2. on the left you can select the date of the change
3. on the right side are all sellers for that product with several usefull informations.