If the number of subscribers to your list exceeds it's limit an alert will be emailed to you.
You can then either reduce the number of subscribers or upgrade your list's limit.
Your account will show the "number of subscribers"/"limit" (eg. 1267/1000) alongside an "Upgrade" link.
Following the "Upgrade" link will allow you to select a larger limit and pay the difference.
The difference is calculated on the list's time remaining until it's expiry date.
If an over-limit list has not been upgraded after 5 days, it is suspended.