What counts?
Every business you are a member of: your own, and each client or team business that invited you.
How do I make room?
Close a business you own in Settings → Close business, or leave a business someone else invited you to. See Can I still get my records after I cancel or close my business? before you close one.
Is each business billed separately?
Yes. Each business has its own plan in its own Settings → Plan & billing. A demo business needs no plan.
Explanation written from the Deline app's own screens and texts (build eba146497 (Deline cf723fe1f)); it has no steps to check.