What happens if I go over the free tier?

Nothing stops working. Go past 10 and new devices still enrol, and everything already working keeps working.

You then need a Direct Debit, which an owner or admin sets up under Organisation → Billing. If there isn't one 14 days after we tell you, your Passbeam account is locked. You can still sign in to set one up, but you can't make changes or enrol new devices until you do. Your network keeps working while it's locked. From then on, every device or person is billed at the published price, not just the ones over 10.

You can see where you stand at any time under Organisation → Billing. Only devices and people actually in use are counted, not everything ever issued.