What happens if I go over the free allowance?

Nothing stops. Right now the allowance is a line we watch rather than a wall: go past 10 and new devices carry on enrolling, everything already working keeps working, and we get told so that somebody can talk to you about it.

That is deliberate and it is temporary. There is no way to pay yet — card payments are not built — so refusing your eleventh device would achieve nothing except stopping you using the product. When billing exists you will be told what it costs before anything is charged.

You can see where you stand at any time on your account page, and the count is of devices and people that are actually in use rather than everything ever issued.