Once you have created your 8th subscription, the 'New Subscription...' menu item in the File menu will become dimmed and you can no longer select it.
In order to continue to create new subscriptions, you need to purchase additional capacity through an in-app purchase. This is easy and inexpensive. The store can be opened from the File menu:
File -> Open In-App Purchase Store...You have the option to purchase additional capacity to hold up to 20 subscriptions or you can purchase the capacity to hold an unlimited number of subscriptions. The unlimited subscription option may be the better value, although its your choice.
There is no need to purchase additional capacity more than once if you have multiple Mac's running LuckNews. If all of the Mac's are sharing the same AppleID, then you can download LuckNews to all of them and share the same capacity you purchased once. To do this, open the in-app purchase store in the File menu as detailed above, and press the 'Restore In-App Purchases' button.