To play Devil's advocate, it may not be a "writer's impression of security" that were behind these scenarios. Hiring extras to fill the shoes of these additional characters costs money not to mention the sets are not generally huge spaces. There are only so many bodies one can comfortably fit in an area before it starts looking ridiculous.
Then there's the whole can of worms that is scene composition. Given the size of TV screens 20-30 years ago if you were to show a ton of people in the shot all at once it's going to look extremely cluttered and your viewers are going to start to complain that they can't make out what everyone is doing.
I think a better use of this discussion would be rather than talking about how many security staff one saw utilized in a show's production it might be interesting to discuss what the general views on security in general and computer security in particular were viewed. For example, in the various Star Trek series, how many times did you see any computer security at all? I can remember the elaborate passwords and voice recognition being utilized when the captain was going to blow up the ship, but think about it. In day to day operations, anyone could walk up to any dang computer workstation they wanted and start pushing buttons.
What's especially amusing to me watching Star Trek these days is a storyline would talk about this computer or that automated satellite installation being compromised by the enemy and not once was computer security was discussed in regards to how it happened (I guess everyone knew that the Bad Guy(tm) just needed jimmy a door open and start typing at the relevant computer console) or how to prevent the breach from happening again. Oh, sure...every week they'd re-program the sensor array to do this or that...something it wasn't designed for or re-program things in engineering to get more speed out of the engines, but when it came to protecting the navigation systems from being taken over and the Enterprise being sent off across the galaxy, it seemed like the only ones who knew how to lock people out were the Bad Guys(tm).