If the number of subscribers on your list exceeds the limit, you'll receive an email alert.
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.
You can choose a higher limit and pay the difference by following the Upgrade link.
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.