Any unique domain or subdomain you scan. example.com and staging.example.com are two.
Any time. Upgrading charges the difference for the rest of the cycle; a downgrade takes effect at the start of the next one.
As often as you want, within the fair-use limits that keep one account from starving the scanner. Recurring schedules are on every plan.
You get a run that says so explicitly. A scan that could not reach the target is reported as failed, never as a clean result.
Every new account starts on a 14-day trial with one website and the standard scan modes. No card needed.