The production secretary, line producer, A.D., or another involved person will number your scenes once your script is going into production.
If you are working on revisions or a production draft and the scenes are already numbered, the question of how to handle renumbering is more complicated, and should be evaluated on a case-by-case basis. John August addresses a specific case [in this blog post](http://johnaugust.com/2007/renumbering).
In multicam sitcom scripts, scenes generally are numbered (or lettered, more accurately), but if you are writing a spec episode, you should consult scripts for that specific TV show to see how it is handled.
* [Renumbering when moving a scene](http://johnaugust.com/2007/renumbering “Renumbering when moving a scene”)
* [The Hollywood Standard](http://johnaugust.com/2007/hollywood-standard “The Hollywood Standard”)
* [‘A’ scenes and ‘B’ scenes](http://johnaugust.com/2004/a-scenes-and-b-scenes “‘A’ scenes and ‘B’ scenes”)