Some people may want to allow 2, or 3, or 5, or 10. I think such a limitation would have to be specified on the package level, but that instead of a checkbox it would be an input box where you can set the maximum quantity any one customer can have.
Then there's the question of whether it's just active ones, or all created regardless of whether they are active or cancelled or pending.
I assume for this purpose, even cancelled ones would be counted toward the 1?