Best to implement the IUpdater interface.
Add trigger during IExternalApplication.OnStartUp for Element.GetChangeTypeElementAddition
The Execute method on that returns the id of the objects that have been created by the user (UpdaterData.GetAddedElementIds) and you can then in that function context find these elements and update their parameters with your numbering script.
If you look in the RevitAPI.chm there is an example under 'IUpdater interface'
There are also likely some samples in the SDK.
Should also say there is no way to embed that process in the family itself, it is done at application level in the product. Obviously you could embed a script in the family and execute it in an open ended updater but that isn't the workflow and probably a poor security choice because your addin would be executing potentially unknown code.
