If you want to check to whom you shared your OneNote notebook, you can go File > Share > Share with people . Here you can only see maximum 31 users not more than that.
If you want to check all users then follow below process:
If you want to see all users then you need to go to the online location of the notebook to check the complete list. Click on File and then info. You will see all notebook and there location with the urls.
Copy the url and open it in browser. Click on share and then manage access. You can see list of users shared with.