Although on rare occasions one might wind up being somewhere and somehow join in the background of a show, in most cases, the shoot is set, well planned, and no accident.
The director has an image of what he wants and if you don’t match, you’re not there.
On rare occasions an Extra becomes a ‘regular’. This is the police station and these Extras are just to be there. The Director wants ‘familiar’ faces.
On most occasions it is a one off.
Many Extras have appeared in hundreds of different shows, from Hit movies to hit shows. But you won’t notice. Either they are blurred in the background, so far back one can’t really see them, and they often look different each time.