There are no specific limitations on tasks.
Here is a section from the Documentation:
Notification tasks can continue to occur regardless of a project’s state. For example, they can continue to occur after a sourcing project ends. In Ariba Contract Management workspaces, notification tasks can continue after a workspace is published, closed, or expired. Notification tasks can also occur when a contract workspace is on hold. If a contract workspace is published, users can complete and create notification tasks without amending the contract workspace.
You can find more about tasks in the User Guide under Creating and Managing tasks.
I hope this helps,
Leslie