Thirty is your monthly limit of listings.
Not of active listings.
If you list 30 items and sell five, you cannot add any more until 30 days/a month has passed.
Go to -> your My eBay dropdown -> top right
Go to->Promotion Offers-> bottom left
Hmmm- I'm seeing Seller Hub for this and I'm not sure if that is what a new seller without a Store will see.
What I see is a report of how many listings I am allowed and how many of those are still available.
You may have used up all your Fixed Price listings but still have some Auctions available for instance.
However, you can contact Customer Support after 30 days and ask for more listings.
Without a Store you normally get 50 Free Listings, once you have proved your bona fides.
And having some successful transactions is a good proof of that.